Knights and bishops are normally the most important pieces in the middlegame. The reason is simple: rooks and queens are more susceptible to incoming fire than minor pieces. Jacob Aagaard, Excelling At Positional Chess
Your practical results will improve once you are more willing to sacrifice material - not for attack (we can all do that) but for defensive and psychological reasons. Jonathan Rowson, Chess For Zebras
Never resign because of an in-your-mind calculation! Too often such calculations have an error. Play the position out! What do you have to lose, except the game? Edmar Mednis, Practical Endgame Tips
When one plans a pawn-offensive on the king's wing one must bear in mind the most important strategic rule in chess: a counterthrust in the centre is the best reply to a wing attack. Alexander Kotov, The Art Of The Middle Game
There is no necessity for Black [in the Evans Gambit] to accept the offer of the pawn. On the contrary, if he retires with his bishop to b6 in reply, he will, as White's last move has in no way furthered his development, gain a small but distinct advantage in position. Emanuel Lasker, Common Sense In Chess
As Black [in the Sicilian], do not give up your fianchettoed king's bishop in order to win a pawn (or even the exchange in some cases) while queens are still on the board, as this will make it much easier for White's kingside attack to score a quick knockout. David Levy & Kevin O'Connell, How To Play The Sicilian Defence