If you're in the UK...
I've been contracting about 3 years now and started it the simple (and probably dumb) way - stick a resume up on jobsite.co.uk, wait for agents to call. Lots will. Be nice to them on the phone but be firm about what rates and locations you're willing to work. You'll get lots of useless ones who haven't even bothered to read it, but no matter, you'll learn to filter them out pretty quickly. Remember the good ones. Rinse, repeat.
I've had two contracts now through reputation, which is quite nice, but getting contracts from previous workmates isn't a panacea. One of them was the most boring thing I've ever done in my life (worse than shelf-stacking in a warehouse) and I quit after three weeks because I was literally unable to complete the work it was so dull. I told the client that I was poor value for money and a recent graduate would be a better choice. The other one was good though!
Also, make sure you're prepared for some time off between contracts, it's pretty much going to happen.