Is your site over optimized? If it is, you probably already know it.Therein lies trouble, if you're having trouble.

Here's a link to a good article and a video of Matt Cutts talking about SEO, Google, and what makes a good site. I think it is very helpful.

Well worth watching. 

SEO is not spam Matt Cutts

PS: Check out all the videos of Matt Cutts on this page. He addresses Page Rank, How shortened URL's work, why a non-optimized site can outrank a well optimized site, and much more. Wealth of content from Matt Cutts.