If you had no regard for your website and wanted it to be penalized or not ranked well, what would you do to make that happen? A WebmasterWorld thread discusses the potential issues that may relate to penalties, and I've turned that discussion into a "what kind of tactics can get your site penalized?" (or "how do I deoptimize my website?")
You can cause problems by considering the following:
1. Have high and unnatural keyword densities 2. Overuse of meta tags 3. Repetitive use of keywords in navigation and linking 4. Removal of non-site external links from within a site (or by adding nofollow to all external links) 5. Having too many reciprocal links
But this is probably just the tip of the iceberg. What could you add to this list?
Forum discussion continues at WebmasterWorld.