Know your profession well; the better you know a profession as a whole, the better you will be when playing that profession in a team build. Also, being familiar with not just your own profession but those who may be countering you can give you that extra edge.
Before engaging a team in battle take the time to scan through their entire team. Knowing what your up against is always beneficial.
Ranged characters should keep close enough to your target so that if the foe falls back, you are still in attack range but far enough so that targeting you is a nuisance.
If forced to retreat from combat, keep your eyes open for terrain that can be used to block your foe's line of sight.
To retreat successfully use skills that increase your run speed or help you evade or block attacks.
Use area-of-effect skills when targets are grouped together to maximize its effectiveness. Don't waste your aoe skills on a single target.
If your opponent gains the high ground try to get to the same height level or cast repeating area effects in their area to make them scatter.
If you are using interrupt skills, try to get a sense of the casting speeds and importance of every spell so that you know when and what to interrupt.
Interrupting your opponents heal can be a fast way to take down your enemy.
Most importantly: Help the team; look out for it before yourself.
After killing an opponent, always keep an eye out for their teammates to attempt a resurrection. Be sure to alert your own team, you must interrupt it!
Stick with the rest of your team. If you find yourself too far away, you could be easy prey for your enemy. Don’t chase... often times your opponents will try to draw you into their team and away from yours, at that time they will turn their focus fire on to you.