The Guardian Online recently posted a compilation of ten rules for writing from a couple dozen writers. Geoff Dyer's are my favorites, principally because of number seven:
"Have more than one idea on the go at any one time. If it's a choice between writing a book and doing nothing I will always choose the latter. It's only if I have an idea for two books that I choose one rather than the other. I always have to feel that I'm bunking off from something."