Pinking the side bushes in botlane can afford your team a lot of extra lane control, as you can be positioned very aggressively without receiving return damage unless the enemy facechecks your bush, which could be disastrous for them both if you're playing a high-CC support like Leona or Taric or if you're playing a high-damage support like Sona or Lulu.
But where is the most effective place to put a pink ward?
This guide assumes a standard lane where the enemy doesn't have any particular stealth that you're trying to ward against other than wards. If there's a Shaco/Eve jungle/mid or a Twitch, everything becomes a lot more complicated and situational, and using a pink defensively becomes more important.
These pictures all assume that you're on blue side; do the analogous things if you're purple team.
Now let's say you did use an explorer ward in the enemy bush, and it's expired already, or that you were able to gain control of the enemy support's bush without placing a ward yet. You want to maintain control of this bush, and to do this you need to clear a ward that the enemy support placed. Where should you put your pink?
This depends on whether the enemy support has a pink or not.
This is where you want to put your pink in the opposite scenario, where the enemy was able to push towards you initially and pink your bush, and now you've finally regained control of your own bush and want to remove their vision. Not that you will necessarily care enough about removing their vision, but if you think that they'll likely push back to you or if your jungler wants to lane gank, then it's important to regain true sight control of this bush.