Rule 47: Know your role, Know your Fellow's Role. In groups, always make sure you know what your class's job is, and what tools you have at your disposal to do it. Make sure you do, in fact, do it. More than this, understand the roles of everyone else around you, and what tools the classes they are playing have to do those jobs.
Rule47a: If you're debuffing, Debuff before you DPS. If you're Crowd Control, make sure everything that is to be controlled, Is, before you do any DPS. Make sure it stays that way for as long as it needs to.
Rule47b: If you're DPS, be sensetive to the roles of the support classes. Be careful of the Crowd Control that's in place, and don't distrupt it. Be mindful of your healers and keep them safe.
Rule47c: Don't expect your functionality to be the same in group work as it is when you Solo. They are often vastly different, and playing as you do Solo, when in a group, is a recipie for wiping everyone, much of the time.