Mistake 3
Existing sites cannot be "SEOed." Stop trying.
Have you ever tried to build a house from the top down? Of course not. That would be silly, right?
The notion may not seem as silly when you hear, as I do, time and again, "We just finished our new site, and we need it SEOed." I don't have a screenshot for this section, as half of the websites I see do not adhere to web standards. (Examples of poor web development are not hard to find.)
A good SEO practitioner can certainly help out after a site is built. But why not employ an SEO strategist from day one in order to ensure the site is being built properly?
The "mashup" web is less about insular content beds and more about adapted content -- content that can be distributed with ease in order to get your marketing messages heard in various environments.
Next time you are working on a new site, get a good SEO consultant involved from day one. Your budget will thank you for it.