All Hail the Oxford Comma

Q: Grammar – everyone’s gotta do it. How important is grammar and what resources do you use to make sure you’re on top of it? When do you break the rules?
 - from Susan
I was in a writing group once in which one writer had a terrific story idea, a historical setting that was engaging, and a cast of characters we loved. But she was afflicted with the worst case of run on sentence structure I’ve ever seen. And worse, she couldn’t see that, seemed not to know what a run on sentence was or why...