Like oWG, I occasionally work on a pay by results basis. And also like him I insist on a percentage of every sale from the moment I start my work. The other option is that I charge a fee for front page positions (and it's a lot more than twice my normal fee, depending on the phrase) which goes into Escrow and is paid to me once the front page position has been achieved and kept for a month.
The client has to pay all solicitor and Escrow fees though - needless to say, I don't get many takers
Any good
SEO will have examples they can show you as well as testimonials from happy clients (happy clients that can be contacted).