Study your games!
I played the following game last night at a local chess club. I had played this opponent (and won) once before. In the first game I played against him, he made three errors. First, he chose a substandard opening with white because, he said, he did not care to study opening theory. Second, he moved very quickly without putting much thought into the position, resulting in a tactic through which I won a pawn. Third, he allowed simplification into a lost endgame, saying after the game that he thought he was ok because endgames are his strength. Did he learn anything from that last encounter? Certainly not. He made the same three mistakes in this game! Further, he made the additional mistake of not keeping score, so that there is little possibility that he will learn from the game. If you want to improve you need to study your games (with ruthless self-criticism) and, if possible, get them analyzed by players who are stronger than you.
Click on "Move List" to see variations and my comments on the game.