It's better not to change entire lines or systems, but to tinker a bit so as to make one less predictable, while retaining the knowledge of the ideas behind the lines. Junior Tay, Chess magazine
A growing trend in modern chess is to know as many different openings as possible, making it harder for opponents to prepare. Simon Williams, Chess magazine
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