I just wanted to state that I have been in a mix of PVP and roulette both as a healer and as a DD. I am now up to 45 commendations. I have gotten 30 of those as a DD role. Mostly they have come from PVP, though I do get a lot of 2's--and once, 3's--from dungeons.
If you are DD and want commendations, things you can do to stand out are:
1) Be nice to everyone and pay attention to your targets.
2) Maximize your damage
3) Help other classes if you know what they need to do to get through
4) Teach dungeons to those who forgot or are new
And most importantly: Don't leave until everyone else has left or 30 seconds has gone by.
MOST of my commendations have come from PVP, I will admit that. If you want PVP commendations, know how to setup a fight, explain (politely) what the strategy is, let people know when they did a good job, and maximize damage. If you are top DD or killer in PVP, you will most likely be getting 1-2 commendations per run.
Good luck and see you with your gilded magitek mounts in a week!