After being shammed by one dodgy SEO company after another, I finally found a decent one, and I did that by choosing a company that did not require contracts. After all, they have to do good work if they want to keep me as a client! They quickly got my website sorted, and my ecommerce website ended up on the front page for many terms inside of 2 months! Very pleased, and I'm still moving up in the rankings. Give these chaps a go, especially if you don't want to risk a contract (and you shouldn't!)

Approximate cost: $3000