Although this post can provide good tips that can be relevant for an SEO consultant or company, some of the tips are relevant also for business in general.

I also liked one of the links in comments, especially the following:

“Any SEO firm that promises rankings or immediate results right from the beginning, is a major red flag.  For one, it is dishonest to promise something that is beyond their control. Basically, claiming that your site will rank #1 or show up on the 1^st page for a specific keyword or set of keywords is not accurate.  The reason is that nobody but the major search engines (Google , Yahoo and MSN) control how websites rank in their organic search results.  No SEO firm can guarantee results because they have no control over the search engines algorithms.  It is true that a reputable SEO firm understands what factors go into ranking websites and can help increase search engine visibility, but no quality firm will ever promise rankings.”