The biggest mistake is to commission a website from a web designer who knows nothing about how users and search engines actually behave. No web designer will actually admit that they are ignorant of these behaviors but almost all of them are.

Only one in thousand new websites are designed for users and search engines, the vast majority are designed only for the client. They may look nice (sometimes!) but they don't actually work. They are not search engine friendly and do not stand a chance of appearing high in the listings. If users do find the site and can't see exactly what they are looking for in the first couple of seconds they will just hit the back button.

To avoid this very common mistake it is essential to find a designer who is up to date on SEO and user behavior studies. There are very few!