I have to say I don't like oDesk as they will tend to go for the cheapest bidder. So cheap that us brits can't compete.
And they have this horrible thing that "watches" every move you make as soon as you boot up and takes screen shots every ten minutes so if you forget to turn it off and you've been looking at naughty things on the internet, images will be captured and stored on your profile for all to see!
Unfortunately thats the case with all such sites these days Maddy. That being said you'll generally find that good projects, with proper funding tend to go with british designers/developers as its better for communication etc.
I also like to remind people if you pay cheap, you pay twice (or something to that effect)
Personally I use business forums to get work, I've never used any of the freelancer sites as I feel that the bidding wars that go on are ridiculous and people are winning jobs on the basis they will do it for $20 etc.
Do a brilliant job for 3 or 4 customers and you'll soon be getting work via referrals. The majority of my work is from referrals and business networking. I don't spend much on self promotion at all.