I'm going to cop out this week and merely refer you to a couple of excellent articles that contain a better discourse on commas than I could ever do. Since I'm reformatting CHOKE for paperback, this is timely. I'm going to refer to this source to get my commas all straightened out!
Both articles appeared in the NYTimes, one in April, and one this month. They seem like companion pieces, so I'll put both links here for your reading pleasure and edification (in case you needed that).
My thanks to Ben Yagoda for both of these fine references.