Well, if I am in an 8-man and I feel that I didn't live up to what I could (Bad day or got distracted) I usually give it to my fellow healer as an apology. Otherwise, I like to give it to a dps that I saw doing a great job (Dodging Titan HM landslides for example).

In 4-mans, I usually keep a close eye on my party and watch for players doing what they are suppose to (SV HM, if DPS notices adds before they get to me and downs them quickly or if DPS are on the ball with Tam-Tara last boss and nothing reaches the middle). I will also give it to a tank that pulls according to the group make up (after looking at gear/class/etc). I have a pet peeve of a tank pulling only after looking at my gear and no one else's ... I know that anyone can do aoe, no matter what the class, but pulling big all the time isn't the best course of action ... I can also flip that when I queue with my Bard friend and BLM friend (all 100+) and the tank pulls 1 mob at a time, lol.