If the answer to the second question is yes, then you should attempt to allin on your opponent immediately. Make sure that you're well warded so that you don't get surprised by your 2v2 turning into a 2v3 or even a 2v4, and be aware of any global ults that the enemy might have, but if you are able to kill your opponents then you should.
Go into lane knowing what it means for you to "win" your lane. Corki against Vayne? You need to be extremely far ahead in gold and exp by the end of laning in order to have won. Sona against Soraka? You're going to be a lot stronger in teamfights assuming the carries are even, so it's ok that you might end laning a bit behind because Soraka can sustain through Sona's poke so effectively. Use this knowledge to decide how risky of plays you should attempt. Yes, if you can easily get an early kill on Kog'Maw, you should go for it, but if it's a risky play then maybe be content to just be even on CS.
LANING IS TRADING RESOURCES.
Think of your lane and the enemy's lane as having sets of different resources. These resources include gold (to be spent on items later), experience, HP, mana, cooldowns, map vision, and lane positioning. Ultimately you're trying to exchange the latter resources for the first two---you want to sacrifice some of your mana so that you can force them into a lane position that doesn't allow them to get gold or experience, or you want to sacrifice some of your HP for all of their HP, killing them so that you can get gold for items later and they don't get experience, etc.
Most people already think of HP, mana, and cooldowns as resources to be managed, but it's important to remember that map vision and lane positioning are also resources. You trade gold for map vision (by buying wards) or for HP and mana (by buying potions). When you decide where and when to use your wards, you're trading between map vision and lane positioning: having a pink ward in the enemy lane bush may mean that you can zone from there, but it also means that you have less map vision and so can't afford to be as risky in your play as you otherwise could with an extra ward on the map that reveals the jungler.
The most common trade is probably between HP, mana, and lane positioning. You trade away some of your mana for some of their HP, and you trade away some of your HP for superior lane positioning. Your goal as a support is to balance the trade between these three resources so that your carry can gain enough gold and experience relative to the enemy ADC that you achieve your previously-decided condition for "winning the lane."
Having lane presence is being a threat. What's stopping them from running past their minions and your minions and pushing your ADC all the way to tower? You are. What's stopping their support from getting free damage onto your ADC while he gets a lasthit? You are. You need to be a threat. And if their support isn't a threat for some reason, then you need to take advantage of that fact. Communicate with your carry about how the enemy is not a threat and then go fight them.
Note that it is always better to run GP10 yellows and GP10 quints with CDR blues and scaling armor reds if your opponent never takes advantage of your weakness early on. Nidalee wins really hard by running all flat AP or MPen as long as you never engage on her. Don't let the lane go the way the enemy support wants it to go based on their champion, rune, and mastery choices. If the enemy doesn't have presence for some reason, you need to take advantage of that. Remember that you become more of a threat not just by getting stronger but also by making them get weaker.
FIGHTING AS A RANGED SUPPORT VS ANOTHER RANGED SUPPORT.
These lanes will be characterized by lots of trading, with pushing your opponents out of lane being more common than killing them. Landing crucial skillshots can result in early (pre-6) kills, but only if you have poked the enemy down enough prior to landing these skillshots. The key is that for the most part you can't autoattack the enemy support without getting autoattacked back, but neither can they attack you without getting attacked back. Thus it will be hard for you to position significantly far past your carry until you have gained an advantage in the lane either by hitting skillshots, taking advantage of wasted cooldowns, full-on engaging together with your ADC when you know you can win the allin, or receiving a gank. Once you hit 6, for the vast majority of ranged supports, both sides' kill potentials increase significantly, and if you hit 6 first then you should try to take advantage of this immediately.
Both before and after hitting 6, you need to be very aware of cooldowns and ranges. Don't waste mana or you'll end up losing the lane just because the enemy support has superior mana sustain, and don't waste cooldowns because then the enemy will be able to run up to you together and either kill you or start to zone you and your carry. Try to position so that you'll always be able to move perpendicularly to the enemy support's skillshots where applicable. If you can get damage onto the enemy ADC, then try to do that, but settling for damage on the support is fine because the support is very squishy and if you can outtrade two or three times in a row then you and your carry can probably allin and kill the support.
When the lane is fairly even, it's important to keep pink wards in the side bushes. The power of being able to walk out of the bush, autoattack once, and then immediately walk back in is enormous, as is the power of a skillshot coming from an unwarded bush. (Make sure you don't autoattack from inside an unwarded bush or you'll lose your advantage of being unseen for a couple crucial seconds.) If one side starts dominating, it's less important to hold a bush that way since the winning side doesn't need to deny vision to be able to zone and the ability to deny vision doesn't snowball the lane very much harder, so you can start to save money on pink wards at that point.
Generally speaking, Soraka, Nami, and Janna want their ADCs to trade while they help their ADCs trade better; Sona and Lulu want to trade themselves; and Zyra and Lux both want to poke with skillshots from a distance, adding a single autoattack to their trade but otherwise not coming within autoattack range. This of course varies a lot with matchup and changes as the balance of power in the lane changes, but Sona and Lulu both deal very good damage from close range (Sona's Q and Lulu's autoattack); Zyra and Lux both have strong long-range poke that they can significantly improve with just a single autoattack; and Soraka, Nami, and Janna don't necessarily trade well themselves (all three have fairly weak autoattack animations) but are able to make their ADCs win trades. These are not hard rules, and if you notice your opponent running such runes so that you have significantly better base stats then you should force more autoattack trades with their support. Other ranged champions that can be played as support like Nidalee, Orianna, and Morgana tend to fall in the category of longrange poke.
PLAYING AS A RANGED SUPPORT VS A MELEE SUPPORT.
At level 1, you are stronger because you can autoattack at range and they don't have enough damaging skills/CC to kill you yet (unless Blitz pulls you). Take advantage of this. There are very few melee supports that can beat a ranged support at level 1. Starting level 2, you're going to have to play a lot more passive unless the melee support wastes a cooldown, which usually happens by missing a skillshot. In the case that the enemy support does miss a cooldown, play as aggressively as possible until that cooldown comes up (you should know roughly what the timer is). Save your cooldowns to disengage a fight in the event that you or your carry gets jumped on.
Bush control is not important for you in this matchup; bush vision, however, is crucial. Don't be the first one to place a pink ward inside a bush unless the enemy support has no pinks. The most important thing for you to avoid is having your own bush pinked by the enemy support with the wave frozen so that your carry needs to walk up to within engage range of your bush in order to lasthit. If this happens, two things need to happen: You need to go back and get a pink ward if you don't have one on you already, and if you don't have sufficient disengage to be able to walk up close enough to the bush to ward it then your jungler or midlane needs to come bot so that you can get the backup you need to be able to push out past this brush. (One thing you can do as Janna in particular is to throw a Q into the bush and then walk up to ward it as the Q is hitting the bush, so that the enemy support is knocked up as you get within warding range.)
Your goals in this lane are to survive and to bait an engage at a bad time (e.g. when your jungler is waiting in the river bush for a gank). In theory, you're only playing this matchup either because you have really good disengage and so your pick of a ranged support was appropriate (Lulu, Janna), because your pick is better for overall teamcomp (you wanted Zyra because you have an Amumu and Galio also), or because you gave another lane the chance to counterpick their matchup, in which case they should be snowballing their lane. In the first case, you should be able to stop their engages and then poke with autoattacks, so you can come out ahead in the lane; and in the second two cases, not coming out behind is basically a victory condition, so just try to meet the first goal of survival.
The way that you typically try to come out ahead in this matchup (after level 1) requires help from the rest of your team. Having a global ult like Twisted Fate, Nocturne, Pantheon, or Shen is huge, as it allows you to play somewhat aggressively when you know that their jungler and midlane aren't near bot, because if they engage on you then their winning 2v2 can easily be turned into a losing 2v3. If you don't have a global ult, see if your toplane can run teleport; the threat alone of having a toplaner teleport to bot can be enough to protect you from enemy engages, even if he doesn't actually use the spell a single time during laning. If you have no global skills, then at the very least try to have your jungler come bot and gank when you bait an engage; hopefully your carry can get a kill or two from this happening and then you can lane much more safely. Note that it's your responsibility to buy pink wards so that your jungler can gank and not the jungler's to know where their wards are and avoid them without your help.
PLAYING AS A MELEE SUPPORT VS A RANGED SUPPORT.
For the most part, at level 1, you don't want to fight the enemy support since you'll take too much damage before you can apply your CC/gapcloser to be able to win the fight after using your skill. You do have superior base stats, and so if the ranged support didn't spec defensive enough runes, you can absolutely start fighting at level 1, but assuming a reasonable amount of armor and MR on the ranged support, you should wait for level 2 to engage---and then engage the instant you hit 2.
Take doubles if you can; interrupt doubles if you can't, and in either case have your ADC push the lane slightly so that your creeps are dying after theirs. This will ensure you hit 2 first and also keep the creep damage advantage on your side. If it's possible that a jungler will show up, then ward tri/river before engaging. You need to win an exchange before you get poked down too much. Usually winning an exchange will mean killing your opponent, but it can also just result in sending the enemy back to base while you push to tower and then go buy yourselves.
Your first goal as the melee support is to not waste a single cooldown. When you don't have cooldowns, you don't have presence, and when you don't have presence, your carry loses. Your second goal as the melee support is to use your cooldowns. If you don't engage, you will eventually get poked down to the point of being unable to engage. And when you're unable to engage, your carry loses. (If you're fighting a Janna or a Lulu or are post-6, engage only if you are able to get past their disengage.)
When are good times to use your cooldowns, then? Generally, if there's not a specific reason to not engage, you should engage. Reasons to not engage include having a massive creep disadvantage, so that you and your carry will be taking a lot more minion aggro than your opponents will if you engage now; being about to hit another level that your opponents already have (but you should avoid this happening in the first place by not letting the wave to push to you too hard); and the enemy having a global cooldown up so that they can turn your 2vs2 into a 2vs3 or even a 2vs4. You would ideally not engage when your ADC is about to get a lasthit, but if you see a good angle on a skillshot that you can definitely hit, then engage anyway.
Bush control is fairly important in this matchup, and as soon as you're a substantial threat to the enemy support, you can get bush control pretty much for free; only ward the bushes if you start to fall behind or if you're pink-warding to deny their vision. A good lane position for you is the one I described above as being bad in the opposite matchup, and if a freeze isn't possible then try to keep the lane at a place where the enemy ranged support can't sit in a bush that you don't have warded and walk out only to autoattack you. Try also to not be at their tower until you're sufficiently tanky that you can turret dive, but anywhere else is fine. Just be careful about engaging into a bush that you haven't had vision of for a while if the jungler hasn't shown recently, unless you think you can actually win a 2v3.
PLAYING AS A MELEE SUPPORT VS ANOTHER MELEE SUPPORT.
Do not engage on the enemy support. If you burn your cooldowns on the enemy support, then a few things can happen. First, your carry can ignore you in which case you've wasted a cooldown and are now unable to protect your carry---or even worse, you are now dead because you're Leona. Second, your carry can engage on the enemy support with you, in which case he will be able to be engaged on by the enemy support, and then the enemy will together be able to kill your carry while the two of you are focusing the enemy support. Your carry can take advantage of the fact that you engaged at all and then focus the enemy carry; however, this will result in a lack of focus overall and the enemy support (if previously positioned correctly) will now be able to engage on your carry. The theme here is that it's bad for you.
Engaging on the enemy ADC can sometimes work out in your favor in these matchups. If you can engage on their carry while their support is out of position so that you can burst the carry down for most of their health, then go for it. If you can't, maybe don't. Crowd control is most effective when both you are stopping their ADC from attacking your ADC and also your ADC is attacking their ADC. This means that if you use your CC when your carry can't attack theirs yet because he still needs to run up, and then their support uses their CC afterwards, when their carry can attack yours and yours can't attack back, then they're going to win the exchange.
Creep advantage is huge in these matchups: if you're a melee support fighting someone with very weak base stats, you only have to be not at a large creep disadvantage to be able to engage safely. However, when your base stats are comparable to theirs, you should have a creep advantage in order to engage safely---while at the same time not being pushed all the way to tower. This means that there's a fairly narrow time during which you want to engage, which is as your lane is pushing out from your tower but before it's gotten significantly past the halfway point in the lane. This of course means that you should be extra careful when the lane is around the halfway point and pushing towards your tower.
Your positioning in these matchups will generally be just within engage range of the enemy support, just outside of autoattack range of the enemy carry, always ready to react to a move from the enemy but not engaging yourself.
Next time I'll talk about roaming during the laning phase.