I can tell you what seemed to work for me. I went from around 1800 USCF to 2086 USCF in about 2 years and I attribute it to the following:
1. For openings just get one or two of the many repertoire books. Pick normal openings, not anything weird. It's helpful to find a grandmaster whose games appeal to you and copy those openings. Do not try to memorize the Najdorf 20 moves deep or anything, just read the repertoire books to get a general idea of the lines. Keep it simple. Most amateurs will quickly deviate from the book anyway.
2. Tactics. One thing I learned from playing a computer is that I suck at tactics. So do you. So does everyone less than 2300. The vast majority of amateur games are decided by tactics. So get several tactics books and just do the problems, whenever you have time. It will take months but if you practice them every day you will start seeing the tactical ideas in your games (at least I did). Repetition is the key.
3. In your opening repertoire you may want to include a gambit or two. You will be surprised how often aggressive play succeeds against weaker players.
4. Just these simple things will get you a very long way.
PS my rating declined a bit after I reached that peak (now it's USCF 1991), partly because I got over confident. But I think the gains were permanent. Now I am trying to figure out if I can ever get up to a 2200+ level.
I have been out of the chess world some time. Now that I am back I find my memory foggy and my tactics lacking. I have minimal time, what are some suggestions for getting back on track?