My team is decently equipped now, so this may not work in the same way for you as it does for me, but this is the way I currently heal:

My priest is holy specced. The reason behind that is simply that single target healing in heroics is a very light job and holy seems to focus more on AE heals, which for boxers in heroics is the most relevant type of healing (IMHO).

I have PoM linked to my dps button, so my priest will cast PoM on the tank as soon as it goes off CD. When pulling multiple packs of trash I add a greater heal when needed. And that's it. I use renew only for healing while I'm moving and only when CoH is on CD. I have PoH and CoH bound to mousewheel down, which takes care of the AE heals and my priest never ever goes below 80% mana (I should probably set him up so he add's some dps).

Again, it helps a lot to have good gear, but I've been running with this setup for a while and when I was running in blues I used pretty much the same setup, except I used greater heal much more often.