I think you might want to target your study a bit - "writing" is a pretty broad field to approach all at once.
Have you had anyone critiquing your work? Are you removed enough from any of it to analyze it yourself?
If so, what are your strengths, and, more importantly for this exercise, your weaknesses?
I kind of agree with blackbird that reading other people's fiction is the most useful way to go, but lots of people read lots of fiction without learning much about writing. I think you need to be able to analyze what you're reading a little as you go. (Since I started writing, it's gotten a lot harder for me to lose myself in a story and just get washed along. Usually I'm too busy figuring out what the author did well and what the author did poorly - I can learn from both).
So I'd recommend figuring out where you are and what you need to work on, then reading some theory books/sites on whatever that area of writing is, then reading some fiction and trying to apply the theory to that work, and then reading your own work to see if you can apply it there.
JUST theory is pretty empty, but I'd argue that JUST reading and writing, with no analysis, will just lead to having more words read and written without much improvement in quality.