The 9 Biggest Myths About Nonfiction Trade Publishing, Debunked. These are all spot-on. I would only add that even when you get a larger advance, it’s typically divided into either three or four installments. So, for instance, I’ve had some divided this way:

First part on signing the contract

Second part on delivering a complete manuscript

Third part on publication of the hardcover

Fourth part on publication of the paperback

You can’t “live off your big advance” when you only have a quarter of that advance — or maybe a third (the divisions aren’t always equal) — during the period of writing.