What are some common mistakes made when trying to implement SEO?

I'm no expert in this area, and I've heard that search engines can be very unforgiving when you try to change or restructure things in certain ways. What should I definitely NOT do? Are there certain things that are wrong but are better left unoptimized if I already have a high page rank?

Below are some points that are the black hat techniques If you have applied any one of these then remove them other wise just keep on building the quality backlinks for you website. and congrats for having the high page rank.

• cloaking and false redirects
• automatically generated doorway pages
• misspelling of well-known web sites
• excessive keyword stuffing
• pages loaded with irrelevant words that do not match the subject
• duplicated content on multiple pages
• hidden text or hidden links
• unrelated and centralized link farms

Yup, and if you are ever wondering whether or not to undertake a particular strategy, ask yourself if it adds to your users' experience.

A few notes: if you are changing domain names make sure your redirects are done properly. Always be aware of potential duplicate content, and I would advise to leave pages with high authority alone. Look at your Analytics tool to see which pages are your most important landing pages and don't mess too much with those.

Just follow the Google SEO guidelines. Do not duplicate content. Do not web spam. Do not link farm.

Matt Cutts blog have a lot of usseful information: Matt Cutts

Better leave robot.txt file part, and don't think of making a keyword rich content, write naturally. Consult an internet marketer for link building.