Improving skills is about deliberate practice, objective analysis (either by yourself or a teacher), and evaluating and fixing your weaknesses. I've been able to improve every skill I've tried with this method.

I consider myself exceptionally good at creating metal music (playing guitar, vocals, recording/mixing/production), and I'm getting pretty good at weight lifting. I am beginning to develop the skill of computer programming, which I expect to take to that level.

For most non-career and non-pleasure skills, I generally stop at the point of diminishing returns. I've learned to cook for myself better than most restaurants, but I don't care to invest the time and energy to become a real artist with it.