Just ran across this blog post by illustrator Penelope Dullaghan (who is also the brains behind Illustration Friday) about what to do if you get blocked on an assignment. It's always so good to hear from artists who are busy at work. It's reassuring to realize that everyone gets stuck, but even more reassuring to get tips on how to overcome it. Read away.
Jillian Tamaki has a great post on a similar subject (scroll down to October 19th, and enjoy the rest of this prolific illustrator's work on the way as well).