oDesk has this work log software they want you to use. You clock in, tell it what you're doing, and it periodically takes screenshots so they can prove you're doing what you say in the event of a dispute. (You don't have to use it, but their payment guarantee only applies to hours logged that way.)

Also, you might want to check out Craigslist. It still has a huge amount of chaff to get through, but most freelancers I know recommend it over any of the "real" freelance job sites.