During the presidential campaign, Peggy Silva, a New Hampshire teacher, submitted the following question to the candidates for a debate: "What don't you know and how are you going to learn it?" This is an excellent question for every writer to ask himself. To improve your writing, you need to know exactly which elements or skills you need to improve and have a plan for improving them. If you have received useful feedback on your work, you should have noticed a pattern of some kind. What weaknesses do your critiquers usually find? Do you tend to have unbelievable characters? Weak plots? Slow beginnings? Awkward sentences? Nonexistent description?
Whatever your biggest weakness is, make a plan to attack it. Here are a few ideas to get you started:
Read books on writing that explain how to improve that element, and then try to apply those techniques to your own writing. You need to stop every few pages and try to put into practice what is being discussed. If you just speed through reading the book, then the knowledge in the book will not come out in your writing.
Find a couple stories that do a great job with that element. Read them each ten times and study them. Authors can't hide their secrets the way magicians can. If you study their stories closely enough, you will see exactly how they accomplished their magic. Examine the way that the author develops the particular element, sentence by sentence. Try to apply those techniques to your own work.
Do writing exercises focused on improving that element. There are plenty of great books filled with writing exercises out there.
Rewrite one of your stories focused solely on improving that element. Then get some fresh critiquers to take a look and see if you've made any progress.
If you ignore your weaknesses and continue to write the same way you have been, chances are any improvement you make will be slow. If, instead, you acknowledge your weaknesses and focus on improving them, you have a much better chance of making major improvements over a short period of time.
Roll up your sleeves and dive in!