I normally try to give it to the person who earned it. Since I'm tanking 99% of the time (ilevel 90 PLD working on gearing WAR now), I can't really give it to tanks obviously (I do CT usually in FC parties and if I duty roulette story dungeons the other tank is usually under geared). Normally I check the two DPS to see who is killing the right adds and see if they are pulling extra adds, also I check to see if healer helps with DPS during the easy pulls and stuff. I also have to see their attitude, if they are polite, and say hi, that's a plus; if they are rushing and complaining about other party members, they will get no commendations.

Also I will NEVER give commendations to people that ask for commendations, especially if they ask right at the start of the dungeon before we even do anything.