I've worked in the call center for over 4 years and that's the best preparation, the best experience that I am still grateful of until today.
I've been working as a freelancer for about 5 years now. It's a tough place to be. But it's probably one of the most rewarding jobs you can ever have.
If you are just starting with oDesk, Elance or any other freelance sites, don't despair. Rejections and declines are normal. Everyone gets one or two of those. But what's not normal is for you to give up just by trying a couple or so times only.
Be patient. Pray. And you'll never know, probably your first client is just writing his welcome email for you now.