Unfortunately there are a lot of people out there that are charging a lot of money with a lot of promises but then not living up to the hype in the search engine optimization industry.
Before you hire someone to do you SEO you should familiarize your self with what it is and how it works. Ask a lot of questions on what they are going to do. The most important thing you can do is to ask for a list of their previous clients and call them to see if they were satisfied. If they are reluctant to give you any of that information then you should be reluctant to work with them!!
One of the things that I do is monitor the traffic before I start to optimize and then compare those stats to a couple months after I have been working with a site to see just how affective my techniques are.