There are a lot of mistakes to be made. Anything at all that violates the Terms of Service of any search engine is a mistake. Maybe fatal mistake for your web presence.

If you are thinking of signing on with some SEO firm you should check them out as thoroughly as possible. Get references. Get proof of results. They should be able to show you some sites where they have first page listings.

So many people get sucked into the hyper from unethical SEO firms that it has become a somewhat serious problem. There is actually a class action lawsuit pending against one of these better known firms. We'll simply call them Traffic-Losers. One of their tactics is to use software to generate a gazillion web pages all with links back to your main site. They will generally post these on your site. Googlebot sees all these links pointing to your domain and of course lends more importance to your site. Eventually, it takes about a month, Google catches on to what is happening and they penalize your site, or outright ban it, for this unethical method of trying to beat the system.

As I write this I am working with a client who has been through this scenario. What we have done with her is to create an entirely new website for her. New design, content and domain. We are just now launching the site and will be starting from scratch with the Google Page Rank and getting listed. This was our client's choice of path to take, and I believe it will work out very well for the long-term.

As you come up with ideas to market your site, and optimize it, the bottom line of your actions has to be to not do anything that can be in any way misconstrued as a "tactic" to "beat the system". Stay away from cheating, keep your nose clean, use tried and true ethical SEO methods and you should be OK.