How to easily get commendations:
Play PLD, WAR, SCH, or WHM even remotely decent.

How to never get commendations:
Play SMN, BLM, BRD, DRG, or MNK at all.

I make it a point to always give my commendations to a damage dealer, assuming they weren't compete morons, but the 1 commendation limit is pretty depressing. The amount of commendations should be half the party size.