I consider Search Ranking Factors by SEOmoz the most relevant resource for SEO, since it gives you reasons on why do a certain activity, rather than another. It can help you a lot in putting priorities – “this is important and easy to do, while this is neither important, nor easy to do”. And, following this, you know how to take a decision.
From the whole list of Search Ranking Factors, I only took a sample, which (may) help you to do on-page SEO:
Note: Also see the Yahoo! Group on which I present similar issues:IMRo. To join, email firstname.lastname@example.org and reply to the confirmation email.