- Use XHTML Strict. There is a webml doctype, however XHTML doctypes work just fine on modern phones, and you don't have to learn anything new
- Images. Keep them small and to a minimum. You want the pages to load fast, and images tend to be the heavier resource.
- Detection. While providing links on your regular pages to a mobile version is handy, it's best to use server side detection, so you can redirect them to a mobile friendly version. Sites like Canadian Tire, and flickr redirect you to their m. subdomain. How to do this depends on what language you use, but a simple google search on user agent detection in language should suffice
- Viewport. You'll want to take advantage of the meta viewport tag, so that the page view just shows what you want them to see, no extras white space. As an example:
is what I use and it works quite well on the iPhone, and blackberry.