Stoney DeGeyter has written an insightful piece on Search Engine Guide on what you should expect from your SEO consultant. Obviously, depending on your goals, this may differ, but the ideas are the same. He explains that traffic is something to look at, but that's not what the client should only seek. Conversions are obviously important, and so are rankings, but that's not how to prove the effectiveness of the campaign. At the end of the day, ROI is really the most substantial measurement of this kind of engagement.
Stoney mentions that communication is key to make sure that expectations are set and followed throughout the duration of the contract. Read more..