Well, of course, there is much more to on site seo, but, believe me, many of the above suggestions really have shown some good results for my site. Well, I also have an old domain and thousands of backlinks etc. But, in fact, I rewrote ALL of my site's 350+ html pages in order to have a completely valid html site and I really think that makes a very big difference for sure. If you at least make sure that your front page is valid, then it should be accessible by any browser. Even though G doesn't mention this point in their webmaster guidelines, I really believe html and CSS validation is very significant in increasing your site's trustrank, which is just as or maybe even more important than pagerank these days.