Be aggressive, but play soundly. Don't take unnecessary chances.
Make sure every move has a purpose.
If you know your opponent's style, take advantage of it. But, in the final analysis, play the board, not the player.
Don't ignore your opponent's moves.
Don't give needless checks. Check only when it makes sense.
Answer all threats. Try to do so by improving your position and/or posing a counter-threat.
Play for the initiative. If you already have it, maintain it. If you don't have it, seize it.
When exchanging, try to get at least as much as you give up.
Take with the man of least value, unless there is a definite reason for doing otherwise.
Cut your losses. If you must lose material, lose as little as possible.
If you blunder, don't give up fighting. After getting the advantage, your opponent may relax and let you escape.
Never play a risky move, hoping your opponent will overlook your threat, unless you have a losing position. In that case, you have nothing to lose.
Rely on your own powers. If you can't see the point of your opponent's move, assume there isn't any.
Don't sacrifice without good reason.
When you can't determine whether to accept or decline a sacrifice, accept it.
Attack in number. Don't rely on just one or two pieces.
Look for double attacks.
Play for the center: guard it, occupy it, influence it.
Fight for the center with pawns.
Don't make careless pawn moves. In the opening, move as few pawns as necessary to complete your development.
If feasible, move both center pawns two squares each.
In the opening, move only center pawns. Unless the opening system or situation requires otherwise.
Try to develop your Bishops before blocking them in by moving a center pawn just one square.
Develop your pieces quickly, preferably toward the center (especially Knights, which often are "grim on the rim").
Develop purposefully, and not just for development's sake.
Don't waste time or moves. Try to develop a new piece on each turn. Don't move a piece twice in the opening without good reason.
Try to develop with threats, but don't threaten pointlessly.
Develop minor pieces early. King-side pieces should usually be developed sooner than Queen-side ones, and Knights before Bishops.
Develop during exchanges.
To exploit an advantage in development, attack.
In the opening, don't remove your Queen from play to "win" a pawn.
Don't bring out the Queen too early, unless the natural course of play requires it.
Try to give as much scope to your pieces as possible.
Seize open lines.
Develop Rooks to open files, or to files likely to open.
Try to prevent your opponent's King from castling. Keep it trapped in the center, especially in open games.
Try to pin your opponent's pieces. Avoid pins against your own pieces.
Don't capture pinned pieces until you can benefit from doing so. If possible, try to attack them again, especially with pawns.
After castling, don't move the pawns in front of your King without specific reason.
To attack the King, pick a target square around it.
When applicable, pick target squares on the color of your unopposed Bishop. (Bishops control squares of only one color. If you have a Bishop that controls dark squares and your opponent has exchanged his corresponding Bishop, your dark-squared Bishop is "unopposed" on those squares.)
Look for tactics especially on squares of the color controlled by your unopposed bishop.
Try to avoid early exchanges of Bishops for Knights.
Double your attacking pieces by building batteries (two or more pieces of like power attacking along the same line). Put queen and Rook(s) on the same file or rank, and Queen and Bishop on the same diagonal.
Build batteries with the less valuable men up front, unless tactics require otherwise.
Maximize the efficiency of your moves. Play flexibly.
To strengthen control of a file, double your major pieces (Rooks and/or Queen) on it.
Determine whether you have an open or closed game, and play accordingly.
Usually play to retain you Bishops in open games, and sometimes Knights in closed games.
To improve the scope of your Bishop, place your pawns on squares opposite in color to it.
Keep your weaknesses on the color opposite to that of your opponent's strongest Bishop.
Trade when ahead in material or when under attack, unless you have a sound reason for doing otherwise. Avoid trades when behind in material or when attacking.
Choose a plan and stay with it. Change it only if you should or must.
To gain space, you usually have to sacrifice time.
If cramped, free your game by exchanging material.
Trade bad minor pieces for good ones.
If the position is unsettled, disguise your plans: make noncommittal moves.
To gain space or open lines, advance pawns.
If the center is blocked, don't automatically castle.
If behind in development, keep the game closed.
Try to accumulate small advantages.
Try to dominate the seventh rank, especially with Rooks.
Use the analytic method. When you don't know what to do, first evaluated the position (as best you can), then ask pertinent questions about your analysis.