I'm usually healer, so I tend to comm casters that give me mana shift, or mch/bard that use refresh.
If neither applies, I give it to tanks that don't annoy me.
If the tank annoyed me, it goes to the dps that made me smile. Either by being in a silly glam/having a funny name/or just making a joke.
If there's no funny dps, I shrug and give it to no one.

For 8 mans, the co-healer gets it. Unless they were inexcusably awful. In that case, it follows the first set of rules.

As a tank, I give it to healers. No ifs, buts, or maybes. (I always give them the benefit of the doubt, which probably isn't always good)
As a dps, I'll give it to a dps who buffs me up, otherwise it goes to the healer.

Of course, these aren't ironclad rules.