I gathered these bits strong players passed on to me over the years.Feel free to discuss and/or add your own.
Look at what's left on the board,not at what left the board
Always look at the most forcing move(s) first
When your opponent threatens something your first reaction should be: Can I ignore the threat?
When in doubt,move a piece out!
When stuck for ideas look for your worst placed piece and try to improve it's position
Never ever push a pawn "just because"
When up in material simplify.When behind complicate
Castle early (and often 😉 )
Doubled pawns are less of a problem when your opponent has no knights
In the endgame: when down in material try to exchange pawns,not pieces.When up in material try to trade pieces,not pawns
A dark square weakness is also a light square weakness!By placing your pawns and pieces on the dark squares you control,you attack your opponents pieces on the light squares
Never relax!Stay alert even when you're up a queen.Relaxing leads to sloppy play -> mistakes -> blundering away a nice win.
Study the endgame!It will improve every other aspect of your game because you will gain a better understanding of the strength and weakness of the various pieces
Enemy pawns and pieces that advance into your territory are easier to attack