To everything he's said, let me add in a few more things...
- Never assume that you'll get paid on time. Many advances today include a component that's due on publication, but that doesn't necessarily mean that you'll get it on the day the book hits the shelves. The broad rule of thumb that I was taught says that you'll probably end up waiting a *minimum* of six weeks, after a publisher is supposed to cut the check, before your agent will receive said check. Some publishers work faster than others, but even the very best are slow at paying.
- The money that you receive from your publisher will trickle in. Let's assume I sell a trilogy tomorrow, for you, for $120,000 (we all wish, but this is mostly for math simplification). So that's $40,000 per book. You're going to receive a third of the total advance on signing of the contract with the publisher, which means that you'll get $40,000 sometime in 2008. (None of this takes out the agent commission, btw.) Ok, so when do I see that other $80,000 you say? Here's where you start to realize the trickle-down nature! What really happens, mathematically, is the publisher takes that $40,000 per book and divides it into thirds. That's nine parts of $13,333.33 (ignore rounding at the moment)--they give you three of those nine parts on signing, which is where you get the initial $40,000. The remaining six parts get doled out to you very slowly.
Part #5: On publication of book #1. (2009)
Part #6: On delivery and acceptance of book #2. (2009)
Part #7: On publication of book #2. (2010)
Part #8: On delivery and acceptance of book #3. (2010)
Part #9: On publication of book #3. (2011)
So that stellar $120,000 advance you got? You're not going to see all of it until probably THREE YEARS AFTER YOU SIGNED THE CONTRACT FOR THE TRILOGY. This is something you should definitely keep in mind.
Also, you should remember that your agent, you know, the person who helped sell the book? We don't get paid until you do, so we also see our earnings trickle in, which is why most agents work to establish a solid number of previously-sold books that are actively earning royalties.