Obviously make sure any site is xhtml compliant and that pages are no more than 3 clicks deep. Some other aspects are ensure title tag is first line of sode after the head tag, use headings to seperate content, try to use hyper text links over nav buttons or drop down menus, use keywords in your sub domain, ensure meta keywords are relevant and not spammy, use alt attributes with your images, minimise the use of flash and java to help content to code ratio.