Mostly just poke ADC when they are going for a last hit on CS (so probably will choose the minion over hitting you), or if possible, bait and dodge the enemy Support so you can hit them with minimal return damage (or otherwise attack them when their CD's aren't up).
Sometimes not poking really is the right choice. Let's say I'm playing Nami or Janna, and they have a Sona. Sona will always go Spellthief's Edge and her poke is both strong and nearly impossible to miss; I just go Ancient Coin and attack the ADC when it's safe but otherwise stay away from Sona so she gets starved for gold.
If you ever think the matchup is so bad for you that you cannot go even or win, just pick up Ancient Coin and don't let the enemy poke you constantly for Spellthief's gold. If they are a Relic Shield user you can probably take your pick of Coin or Spellthief's.