A small notebook of checkmate patterns, opening traps and things I keep forgetting.
Classic patterns every club player should recognise instantly. Find the move, then check.
Most back-rank accidents happen in the first moment of calm after a long tactical sequence. A cheap habit: once the opening is over, spend one tempo on h3 or g3 (or …h6 / …g6) before you start pushing for something. It costs almost nothing and removes a whole family of tactics from the board.
Before every move: what are all the checks, all the captures and all the threats — for both sides? It is slow at first, but after a month it becomes the way you look at a position.
The Giuoco Piano looks quiet, but the plan with c3, d4 and a slow kingside build-up gives plenty of play without memorising twenty moves of theory. Good choice for rapid games.