So where to start? Sewing books. No particular recommendations here, but among the books in my own sewing library I have Singer Reference Library The Perfect Fit, Palmer/Pletsch Fit for Real People, Nancy Zieman's Fitting Finesse, Bodymapping by Kathy Illian and Shortcuts to a Perfect Sewing Pattern by Rusty Bensussen. The last two are more about creating slopers but are still useful for fitting help. Obviously, there's a wealth of information on the Internet. On the Silhouette Patterns site you'll find an article on Understanding Fit as well as several webcasts. Go to Pattern Review and search their knowledge base or their boards. Subscribe to Threads, buy their archive, check them out online. Just some of the more obvious suggestions. Most of all I think I really profited from attending sewing shows, both Martha's Sewing Market in Arlington, Texas, and the ETA here in Dallas. You'd be amazed at how much you'll learn from a 30-minute class, given by excellent instructors such as Peggy, Cynthia Guffey, or Connie Crawford, just to name three whom I found really helpful. If there's a sewing show you can get to, go. You'll find it informative, inspiring and lots of fun.
For those of you interested in more specific aspects of how I refined this pattern, and photos thereof, please continue.
Here is a comparison of my altered pattern pieces and the original pieces of McCall's 4309 for the jacket.