The most important thing is obviously keeping it systematic and regular. Depending on how much time you have I suggest you to split it this way:
- 15% solving patternal tactics. Online or even better - get a puzzle book featuring easy puzzles grouped by topic to improve your patern recokgnition. There is also
blitztactics.com which is great
-5% hard attack puzzles. Get a book of puzzles which do not have a straightforward, forced solution, but rather requires great attacking capabilities. It improves your practical attack understanding (how to get all your pieces to attack, which files to try to open, what defending pieces you should exchange in what situation).
- 15% work on opening theory. Keep it regular! Repeat your repertoire lines daily! I do not think there is a better way for this, then just getting chessbase and browsing through opening book, choosing what variations you like, saving them and repeating them later on. Also make sure to watch some superGM games to learn plans of those openings.
- 10% work on endgames. Get silmann's complete endgame course and download some puzzles (I got some pgn myself). From time to time read a page or two and solve a few puzzles.
- 20% work on general chess principles, strategy, pattern recokgnition and so on. You could try reading the reasses your chess, logical chess move by move, my system, chess for tigers, improve your chess pattern recokgnition and so on. I also like the reasses your chess workbook. I have great times in he evenings "solving" puzzles in it.
35 to 50% playing. Aha. Not bullet, but at least 10+0! And do not forget to analyse your mistakes afterwards.
This is system my students and I myself had a lot of success with. Obviously depending on how much you play chess for fun rather than being good you can tweak this as much as you want.