Tip 1: don't play Bg5
The first tip I want to give is about the opening. Possibly I should have started with endgames, but it doesn't matter. There is a famous piece of advice from Lasker (it appears in Common Sense in Chess) which says: don't play Bg5 before the opponent has castled. But grandmasters play Bg5 all the time. Can it really be that bad?
Of course, it's not necessarily bad, but Lasker's advice is quite good. It's certainly a bad idea to put your bishop on g5 automatically without thinking about it first. And, in a 3 minute game, one thing you don't have is the luxury of thinking very much - especially in the opening.
So why isn't Bg5 OK before the opponent has castled? Simple: black can chase it with h6, and then after it moves back to h4, g5. But doesn't h6-g5 weaken black's king? No, because black hasn't castled. That's the point. So black has got two free moves for nothing. And if white happens to have castled kingside, and even more if the centre is already blocked, black has a ready-made attack with h5-h4, g4 (if the g-pawn hasn't been taken), 0-0-0, rooks to h8 & g8, etc. Furthermore, with the bishop on g3, white is in even more trouble, because later on black will be able to attack the bishop again and gain even more time.
But what if white takes on f6? After all, isn't that the point of Bg5? Well, certainly it's possible. But then if black hasn't castled yet, there is the possibility of gxf6 and a free open file pointing towards the white king.
However, note that none of this applies if white hasn't castled kingside at all. So is Bg5 OK in this situation? Usually it's fine, but see the game Fluss v Nimzowitsch, corres. 1912, for an impressive example of what can go wrong for white.
Oh, and one more risk of an early Bg5, this time especially in queenside openings. There's always the possibility of the bishop being captured after the opponent's Qa5. Do not underestimate sideways queen moves! They are hard to spot. So if you are tempted to play an opening like 1. d4 Nf6 2. Bg5, stay vigilant; a bishop on g5 is never an entirely comfortable piece.