Write correct semantic HTML
Use a strict doctype
Define your document language
Define your content type for the XML and once more in the head
Use microformats where possible: hCard, hReview, XFN, etc.
Validate your finished page (or have reason why it does not)
Correct HTML elements for the right content, did you know the <samp> element?
No browser dependent hacks (if possible)
Use only external stylesheets so it can be cached (no more then 8kB)