Always be wary of old theory in very sharp positions. There may well be a dangerous improvement lurking which computers will quickly spot. Richard Palliser, Starting Out: Sicilian Najdorf
It is not sufficient to be a master of one opening. You must also be a jack of all the rest! Sure, go ahead and learn one defence thoroughly, but also get a feel for the others. Force yourself to play through master games or play some blitz games. You only require some general knowledge of the general principles. John Emms, Chess For The Rank And File
When your intuition tells you that there should be a forcing combination in the position, but your concrete analysis can't make it work, try the brainstorming technique of reversing the move-order. Lev Alburt, Chess Rules Of Thumb
The creation of an operation base for the knight is one of the most important motifs in chess strategy. Luděk Pachman, Complete Chess Strategy 1: Planning The Pieces
The formation and carrying out of a plan of action is the main task in the course of the process of the chess struggle. Peter Romanovsky, Chess Middlegame Planning