Play a lot of long time control games.
Read books on the fundamental areas like strategy and endgame.
Regularly solve tactics puzzles.
Memorize a few main lines for the openings you play, but don't fall into the common trap of focusing a majority of your time / effort on openings. Beyond knowing some fundamentals about the openings you play, opening study will not improve your results.
I'm brand new to chess. I mean, I've played chess before, but I've never been very good. How do I get better?