Few sacrifices work if the attacker cannot bring several pieces, particularly the queen, into the attack with great speed. David LeMoir, Essential Chess Sacrifices
There will always be times when your preparation or memory fails you (in the opening) and you have to make it up. In these cases you should aim to find a move which is sensible on general principles and which does not have an obvious refutation. John Nunn, Grandmaster Chess Move By Move
Don't just study variations in your repertoire; investigate the typical endgames that can occur from different variations as well. Andreas Tzermiadianos, How To Beat The French Defence
Never forget that your opponent is always thinking of ways he can hurt you. The time to relax is AFTER the game. Samuel Reshevsky, The Art Of Positional Play
Quick rule for plans: look for your least-active piece or pieces, and then look for a plan to make them active. Cecil Purdy, The Search For Chess Perfection
It's not most players' cup of tea to win an early pawn (as White) and then try to grovel out into an ending. Playing White should be more fun than that! John Emms, Attacking With 1 e4