Most poker sites have a chime that sounds when it is your turn to act. When this chime sounds, count down in your head "3...2...1..." at the same pace every time before you raise, fold, or call.
This will eliminate a key online tell: betting timing. The speed at which you bet can cue your opponent to the strength of your hand. A good opponent will get a subliminal feel for the speed at which you make bluffs and legitimate bets. Poker pro Daniel Negreanu said he has played many online opponents where he could tell the legitimacy of their bet based solely on betting speed.
In addition, the constant counting down provides constant stimulation to the logic center of your brain. This helps you remain calm and will reduce tilt and bad decisions. You will probably even find there are a few occasions where you reconsider that bad bluff or call by the time by the time you have counted down.