Study tactics and endgame, definitely.
Train yourself to set up good positions.
Learn to adapt.
Play often against a variety of players.
Don't fear strong players
Don't have too many games going on at once. My limit is nine (9) at a go.
Learn from wins and defeats.
1400 to 1950
The French Defense!
Well, not only that, of course. But the gain in positional understanding that came with switching from the Sicilian to the French augmented the benefits of working through many thousands of tactics problems and playing through thousands of GM games.