I saw a Website that raised the issue of how to choose a Search Engine Optimization firm. It looked very promising, but in fact, I didn't find it particularly helpful. So I wrote an article of my own. The meat of that article is 11 things you must know before choosing an SEO firm. Why 11? Because it wouldn't fit in 10. The most important things:
1- Understand when a Search Engine Optimization Firm can't help
2- Understand what things you need to do yourself.
3- Make sure you know exactly what SEO is
4. Make sure you know exactly what this SEO consultant is going to do
The rest and some comments on that other site are here Selecting a Search Engine Optimization Firm