The English Opening is basically geared to controlling the d5 square. White starts off with 1.c4, follows up with Nc3 and completes the job with a kingside fianchetto. Gary Lane, Ideas Behind Modern Chess Openings: Black
Stockfish is a wonderful tool, but remember: it's not enough just to notice that the machine recommends a move - you need to understand why. If it's not immediately obvious, play a natural move for the other side and keep digging until you'd be able to explain and demonstrate to another player why the machine evaluates things the way it does. Andrew Greet, Chess magazine
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