Study Up

It never hurts to get a few writing books and study up on what you feel is a weak point. There are plenty of good books out there that cover everything from grammar and usage to plot and character development. If you don't want to actually purchase the book, look into getting them from the library. By reading books about writing, and not just books in your genre, you can improve your writing technique and spend more time adding details and less time worrying about comma placement.