There are potentially 100's of variable factors you need to keep in mind, but as always it is important that you make the website fully accessible, make it validated.
Some of the points to keep in mind:
- title tags for all links and images
- META tags which are relevant to the PAGES content not the WEBSITE content
- a good text to media ratio
- structured content using h1, h2, h3 tags (never more than 1x h1 tag per page)
- indepth interlinking navigation which allows multiple ways of accessing certain pages
- sitemap both .xml and html format
- robots.txt (to disallow access to potential pages which don't work)
- SEF urls i.e. instead of www.mydomain.com/gallery.php?pictureid=23 use http://www.mydomain.com/gallery/23-m...john-in-paris/
- ACCESSIBILITY (can't stress this enough, just by using good practice the SEO will come with it in hand to a certain extent)
Hope they help, some of them are 1 and the same but it gives you an idea of what you are doing and some of the aforementioned tactics will not work without other parts of the list being intact, as well as some of them aren't entirely proven to help SEO but will hinder it if not done!