I'm in the midst of an online course in self-editing taught by Carina's executive editor, Angela James. She covers everything from the most basic basics of punctuation and word usage to more advanced topics of voice and craft, with a goal of helping both published and unpublished authors submit more polished and professional manuscripts.
It's hands-on, so we're encouraged to apply the lessons to our own manuscripts. Though some of the basics aren't really issues for me--Mrs. Denney and Mrs. Atchison taught me well back in my school days, so I rarely struggle with grammar, punctuation, and capitalization--I've already learned MS Word tricks I'd never heard of before. And there's nothing like the executive editor of your publisher telling you to read your manuscript out loud to make you actually DO it and discover what a useful tool it is.
I strongly recommend Before You Hit Send to beginning and advanced authors alike--and you can take it beginning tomorrow at Savvy Authors (membership required). It's $35, but well worth the price of admission.