Well, I can only tell you what worked for me.
1. Get a .22 pistol. Mine is a Ruger MkII.
2. Shoot every week, at least 100 rounds.
3. Focus on the basics (already enumerated in prior posts).
And for me, probably most important:
4. Aim small, miss small. Ever since reading that, I've been using 3x5 index cards as targets. I stick a 1" orange dot in the middle, run it out to 15, 21, 25 feet, and concentrate on that little dot on that little card. I'm convinced that has improved my shooting more than anything else. I am nowhere near world class, but my improvement over the last year or two has been noteworthy. And it translates directly into my revolver and 1911 shooting.