Janet Meiners Thaeler identifies strategies that might classify a SEO company as dealing in "black hat" -- deceptive or unethical SEO practices. These techniques include keyword stuffing, or repeating "words or phrases so often that it's unnatural to read," and overusing bolded links. Such techniques, and companies who use them, should be avoided. While black-hat practices might get your site to the top of a page in the short term, long range it could get your site delisted from search engine rankings.
Also, companies that guarantee higher rankings on competitive search keywords should set off alarms. "It's important to remember that SEO companies do not control search results and therefore can't guarantee anything," Thaeler writes. "Instead they can take specific actions to create the best chances of ranking well amongst the hundreds or thousands of other sites."