the trick to get commendations is not to play as best as you can, you have to play how the others think you should to xD

i noticed (as a heal) you will get LESS commendations if you do damage, even if nobody dies. if you do damage the tank thinks probably you don't heal him enough and in emergency he will die and the DDs can think you want to tell them that their damage is too low xD (what do you think if a summoner / blackmage starts spamming his heal?)