The only real mistake I made was trying to over-invest in planning for it. I dithered around trying to make a perfect screencast video showing how to use our web-based data entry tool, etc, write perfect decision notes, etc. This was all a waste of time. It would've been better to just jump in sooner.

You'll meet a lot of people who can't do what you want, and as soon as you post a project you'll get a lot of bid-spam. One trick I had was to make the instructions on how to bid slightly non-trivial. We had 2,000 documents to get done, and didn't want to give it all to one person. So I instructed bidders to bid for a number of documents and a price per document. This meant that if someone posted "I will do it for $100", their bid was meaningless, and they could be excluded from consideration.