The one general rule I got by:
Give the commendation to those who are the most deserving of it.

Role is a largely secondary concern, but can help with a tie-break. I do try to give one out every run, with exceptions being cases where either it's clear absolutely no one deserves a commendation or everyone runs out so quick I don't get the chance to give it to anyone.

Here's a few criteria for me:
  • Be kind and courteous. Jerks NEVER get a commendation from me, period.
  • Be helpful. Giving out advice, helping people with mechanics, that sort of thing.
  • Be talkative. Silent runs are dull. Amuse me, and it'll provide some encouragement.
  • Pull off a feat or "moment of glory". Do something impressive, and you'll earn it without question (aside from my exception in the first rule). Preventing a wipe with a well-timed rez (especially healer LB3), clutch healing as PLD or RDM when the healers are down, fast pick-up of the boss as an off-tank if things go awry... that sort of thing.
  • Generally do a good job. This applies to all three roles, doing it well is expected and will be given due credit.
  • Playing a role which others don't want to. In other words, if all else fails, I admittedly do favour tanks and healers. I mostly play a tank, so I frequently default to healers; if I'm DPS or healer, I will default to the tank (provided they don't do the sort of tanking I despise). DPS are, from my perspective, usually just "along for the ride"... so they have to rise above and beyond just being there and doing what they're supposed to do to earn a commendation. Many actually do make this effort.

Perhaps a bit biased, but a huge part of the part of commendations for me is to be a team player and provide your best efforts. Sure, there's times you save the day and you get nothing for it... but there's just as many times it comes back and you earn what you were owed with interest.