the infamous megnut has written a very nice piece on using personas in designing a web application. it sounds like a great technique. one other trap that i think people fall into is making something overly pretty, or optimized for first-time use, that make it a bear to use on a regular basis. it seems that thinking about how a web application fits into the long-term work habits of a persona would help avoid that trap. there's some interesting thoughts in the related discussion, too.