The best way to improve on your writing is to write, then go back in a few days and re-read what you've written.
A second way is to write, and then ask an outside writer, proofreader or editor to give you feedback.
A third way is to write and submit your work to a site that screens it through an editor. Then take heed to the feedback, if any is offered.
Blogging is another great way to grow your writing skills. You are nudged to update your blog, and thus write.
One suggestion of how not to test your writing skills is through contests. Many contests are set up to earn money off new writers who pay an entry fee. Your piece may not even be read. Look for other ways to have your writing viewed and then try to sell your writing, not the other way around!