Don't hire an SEO company that puts links to their other clients on doorway pages, offers to sell keywords in the address bar, or has had domains removed from Google's index. Janet Meiners Thaeler tries to take the guesswork out of finding a good SEO company. She encapsulates a few tips from Google for companies looking for a SEOer to help get your Web site in top shape and then adds a few of her own.
Thaeler suggests making sure you understand the SEO company's strategies and specialties before hiring. Common sense says that if they've branded themselves a "social media expert," make sure they have the reputation and experience to back up the claim.