On Problogger.net there is a great post on improving your blogging through practice. Darren compares it to tennis, but I think it is just like anything in life, if you apply the right amount of time and effort the rewards speak for themselves.
1. Action/Reflection
2. Set Yourself Assignments
3. Write for Different Mediums
4. Write as a Guest Blogger
5. Ask for Critique
6. Read and Analyse others
7. Speak Your Posts
8. Critique Your Own Old Work
9. Get an Editor
10. Take a Course