I give commendations based on these factors.
1) Tank did an awesome job.
2) Healer did an awesome job.
3) DPS did an awesome job.

In dungeons my commendations usually go between the tank or healer if they do something to make the run very smooth. For DPS I give it to them if they LB when they were supposed to or just managed to save us when things went south. In larger party instances I am more willing to give it to DPS that I see do something nice. I also give my commendations to those who give actual helpful advice rather than complain.