This is a compilation of stuff Matt Cutts has said historically, minus some of the more recent stuff here, here, and here. I decided I’d dig backwards and document some of the older stuff. I dated it accordingly. Here it is:
Matt recommends using dashes over underscores to delimit words in urls. 2005.
Google does not algorithmically penalize for dashes in the url despite the fact that some have raised it as a possible heuristic for spam detection. I think WordPress pretty much precludes this anyway. 2005.
Google updates their index data — including backlinks and PageRank all the time. However, they export and publish new backlinks and PageRank data approximately every three months. New backlinks and PageRank are meaningless — it is not an update. The information is likely already factored in for awhile before you see it. 2005.
Hiding text using similar colors and background colors can actually be worse than using the same colors. Using “#EEEEEE” instead of solid white on a solid white background can look worse — as if you’re trying to hide it. I suspect this is a heuristic for detecting hidden text. 2005.
If you sell links, Matt says you should use link condoms. Otherwise your reputation may fall. I assume this means they will devalue your outbound links. 2006.
Googlebot can only crawl the free portions that non-subscribed users can access since it does not log in. Therefore, be sure to excerpt material in the free version that that offers value. Vanessa Fox. 2006.
If you must use Flash, you must also make an HTML version available as well. Block the Flash version from the crawlers with a robots.txt file. Vanessa Fox. 2006.
Use user-friendly URLs like “african-elephants.html,” and not “343432ffsdfsdfdfasffgddddd.html.” Don’t overdo it either — african-elephants-and-their-habitats-etc-etc-etc-etc.html. Vanessa Fox. 2006.
Assign unique, descriptive <title> tag and headings to every page. Vanessa Fox. 2006.
Minimize the number of redirects upon hitting a URL. Vanessa Fox. 2006.
Minimize the number of URL parameters — 1-2 parameters if possible. Vanessa Fox. 2006.
Don’t use a parameter named “id=” in a URL for anything other than a session ID. Otherwise, it may not be included in the index. Vanessa Fox. 2006.
Earned-links are earned and given by choice. Google does consider buying text links for PageRank purposes to be outside our quality guidelines. Vanessa Fox. 2006.
Blackhat SEOs may be leery of using Google for analytics, but regular site owners should be reassured. Vanessa Fox. 2006.
Google’s is against selling/buying links, and Matt indicates they are good at spotting them — both algorithmically and manually. Sites that sell links can lose their trust in search engines. 2006.
Google is focusing on detecting spam in other languages in 2006 — Italian, Spanish, Chinese, etc. 2006.
External (domainA -> domainB) 302 redirects are largely treated as 301s now. 2006.
Canonicalization is the process of picking the best url when there are several choices, and it usually refers to home pages — www.example.com vs. example.com vs. www.example.com/index.html. Since all these urls are different, a web server could return completely different content for all the urls above. When Google “canonicalizes” a url, it tries to pick the best one and elimintes the others. To help Google, link to resources on a site consistently, and use 301 redirects to enforce it. 2006.
Do not use the URL removal tool to remove domain.com if you are worried about URL canonicalization and have both domain.com and www.domain.com in the Google index. This will remove the entire site. 2006.
Search engines can perform canonicalization for things like keeping or removing trailing slashes, upper vs. lower case, or removing session IDs from bulletin board or other software. 2006.
404s (Gone, but may reappear) are treated the same as 410s (Gone, but will not reappear). Most web masters use 404s as 410s anyway. 2006.
Following these tips will probably help you rank better, so long as you actually trust Matt Cutts. I’d assert it’s wise to approach some of his advice with skepticism, since I’m sure Google gives him guidelines as to what he can actually say, but following the advice above won’t get you into any trouble.