I try to award commendations based on who is the most helpful, or the most chatty (good chatty, not berating people chatty). If no one really talks then I look at how people are preforming. Are the DD's attacking the right mobs and moving out of stuff, is the healer constantly active, is the tank marking and reasonably holding aggro/using cooldowns... I have had a few instances where the person I wanted to give a mark to left before I had the chance to do it.