I know several people who simply choose a search engine optimization firm based on a quick glance at their website. However, you want to choose a search engine optimization firm that works well with your business model and can get you the results that you need. These companies are not all equal, and if you are not careful, you could end up spending a lot of money for very few results.
One of the first things you will want to do is ask the company to see case studies of their clients. Sometimes they will even provide you with contact information for one of the clients so that you can speak with them about the search engine optimization firm. When you are looking at firms, make sure to get the best fit.