There is a great article on Focus Internet Services website about choosing an seo company and making sure they have your best interest in mind. When you are looking over the work that a search engine optimization firm claims to have done, all the links they have built and all the content they created...if you can't find a credit on the work anywhere then ask to see the client list and ask for contact names. Calling the contacts to ask if this company really did a good job on the seo is probably the best way to make an informed decision, because half the time the work in the portfolio doesn't even belong to the company. There are cases out there of companies being fired for doing a terrible job, only to have the company in their portfolio and claiming responsibility for work that they didn't do correctly. Other companies had to clean up the mess and they are still claiming they did it!