For a good fit, worry about the back.
The good news is that I'm a curvy girl. The bad news is that my hips are 20 inches wider than my waist. That's a huge fitting challenge, pun intended. Basically, this guitar and I were switched at birth. :-)
According to Kathy, if the crossback fits, the sweater fits. Who knew? Make sure your sweater back fits well between your shoulders and your entire sweater will fit better. In my case, my crossback measurement is 18 inches, so I knitted the upper back to be 18 inches.
The crossback isn't the only key for a good fit, however. Kathy Zimmerman says if you want to make a sweater that fits, do back neck shaping. Many designers skip this shaping because magazine editors insist on dumbing-down patterns for novice knitters. But a gentle curve at the neck lets the sweater sit comfortably on your shoulders and drape gracefully to your hips rather than riding up in the back. In fact, Kathy said I should stop making in-the-round, seamless yoke sweaters and make garments with side seams, set-in sleeves, and back neck shaping instead. She said I'd get a much better fit. I'm sure now that she is correct, but I love round yoke sweaters and will keep on wearing them anyway.
The original Augusta pattern does not have back neck shaping but I added it and have a better-fitting sweater as a result. I used Sweater Wizard to figure out the decreases (more on how I used this software in a forthcoming post). Knitting the back neck shaping was easy; see this great video from EasyKnittingDesign.com to learn more.
So, to sum up lesson 1: for a better sweater fit, focus on the only part of the sweater you've never even considered - your back.