Lots of good advice, just one tidbit to chuck into it:

In the contract (yes, build one, quadruple-check the text on it, and get it signed and notarized), put in 'milestones'.

This way, you get paid for so many hours and/or every so many panels completed. No further work occurs until each milestone mark is paid-up... NO EXCEPTIONS. Any delays caused by non-payment will result in an idle fee of $n per day.

An up-front retainer (say, 25-50% of the estimated total) is a solid idea as well. Finally, make sure that revisions will be expensive after the characters are approved (at a rate of $x per hour), for each panel.

Wanna be a pro? Charge like one. Note that I didn't say "charge as much as one", but break it down the same way. If the guy is serious, he'll do it. If he isn't, he'll have a very hard time finding easier terms with anything that can be considered pro-grade results.

/P