The best links have your keywords in them. Links from sites that are listed in Yahoo and other large directories are the most valuable. If we have helped you, please make a link back to us. If you manage to get one of your pages into the top 20, leave it alone . Additional tweaking of a successfull page is much more likely to decrease its rank than raise it.
Density can be too high.
Many search engines set an upper threshold to eliminate spam sites. Study the individual search engines. With that being true, many people believe that allowing a search engine to find your site with its spider is far better than submitting the page. This is the philosophy I use and I have many high rankings for competitive terms. The trick is to make sure your site is listed in Yahoo.