Bullet Tips

1. PLAY A DEFENSIVE OPENING AND USE PREMOVES : Playing a defensive opening allows you to premove and save some time.  Playing an offensive opening requires you to think about tactics, and you can easily get checked, which loses time. 

2. PLAYING A CLOSED POSITION : Playing a closed position allows you to premove while you think about what move you want to move next. Don't premove too many moves ahead. If you do, your opponent could premove a pawn or something and be able to win material. 

3. PLAYING AN OPEN POSITION : When playing an open position, try to put your rooks on the open files and bishops on the open diagonals. It also would help if you trade off your knights for your opponent's bishops. Bishops are generally better than Knights in open positions. Usually in an open position, discovered checks or unexpected checks can stall out 2 or 3 seconds. 

4. THE ENDGAME : If you manage to get into an endgame in a bullet game,  most of the time both sides have less than 20 seconds left. When in an endgame, always make sure you have at least one pawn. That way, if your opponent times out, you get a win instead of a draw. The best way to delay your opponent's time is to check your opponent so they will be forced to move their king. Another way to delay the time is to cut off your opponent's king's squares. Try cutting off as many squares as possible so the opponent cannot keep premoving his king.

Credit to Mathgenius1157 for the tips, I'm resharing it.