I am a staunch advocate of "less is more." My observation is that at every level of the game, players are using too many conventions. Too often I see players (from beginner to world champion) misusing or forgetting their methods. Everyone would benefit if they would just KISS. (Keep It Simple, Sweetheart). Don't spend all of your time learning new conventions instead of working on your play.
If I had to choose just 4 conventions, this would be my list:
1. Negative D