1) When you see a good move, sit on your hands and see if you can find a better one.
2) In chess, if you learn to consistently (each move) do the little things: take your time, count the material effect of your move, and check for basic tactics, you will soon find that these are not so little!
3) If you get way ahead in material, it is more important to use all your pieces, kill your opponent's counterplay, and safeguard your King, than it is to try and get further ahead.
4) When looking for tactics - for either player - look for Checks, Captures, and Threats, in that order - for both players.
5) Never, ever assume that your opponent has no threats, even in the most lopsided positions.
6) Concentrate. Keep your attention on the board. Don't let your mind wander and don't you wander either. Don't leave the board unless necessary.
7) Respect all opponents, but fear none.
8) Focus on playing your best, rather than on winning. The wins will follow.
9) When even or ahead, play hard. When behind, play harder.
10) Keep the normal value of the pieces in mind (queen=9, rook=5, bishop=3+, knight=3, and pawn=1), but remember that these values vary according to the position, mobility, and potential of the pieces. Whether attacking or defending, count the number and consider the values of both attackers and defenders on a target piece, pawn or square before exchanging or occupying, to insure against losing material.
11) Determine the purpose of each move by your opponent. Ask yourself, "What is the THREAT?" and "What has CHANGED in the position?" after each of your opponent's moves. Concentrate on offense and attacking, but recognize and answer all threats.
12) To win a game of chess, you must first not lose it. Avoid mistakes, such as leaving pieces en prise (unguarded) or exposing your king. Before each of your moves, ask yourself, "DOES THIS MOVE IMPROVE MY POSITION?" and "IS THIS MOVE SAFE?" Avoiding mistakes is the beginning of improvement in chess. THINK before you move!
13) The two most common (and often fatal) mistakes in chess are moving too fast and overlooking opponent's threats. Sit on your hands until ready to move
14) Make sure EVERY move has a purpose.
15) Answer all threats, but do so while trying to improve your position and/or posing a counter-threat.
16) The goal in chess is to play the best move in every position.
17) Play slowly. Haste and carelessness are greater enemies than your opponent. Accuracy, not speed, is essential in chess. Be patient. The reward for speed is a legacy of lost games.
18) Players usually make their worst oversights in dead won games or in dead lost games. It is surprising how often a mate in one is overlooked when one's position is already hopeless or when you are winning easily.
19) The best practical rule for a winning game: destroy your opponent's counter-chances. It may be slower, but it's surer.
20) When forced to choose among moves, play the most necessary one first.