Hiring that hipster from San Francisco?
Not doing any SEO at all?
I say the biggest mistake is this: Trying to rank on the first page of Google for a search phrase that's too competitive. You can avoid costly publicity and link building campaigns that are required to rank if you find ultra low competition search phrases to write about. These often require few links to rank, and occasionally no links at all.
This article is a technical guide that explains how to find LOW COMPETITION search phrases. Do this and you will appear on the first page of Google's regular search results more often.
If you want a little background first, here's a Moz article on keyword research, and here's one from Brian Clark.