1. Use 100% valid XHTML 1.0 code. (Note: it is likely that WML will be phased out in the near future.) Many optimizers on the traditional WWW do not consider using valid code as a best practice. Mobile search engines however may have more trouble digesting invalid code. Be safe! If you validate, mobile search engines won’t have any trouble with your site.
2. Follow accessibility best practices. These will ensure that your content is accessible to anyone regardless of their platform — this includes mobile users and mobile search engines. The W3C’s Web Accessibility Initiative is a good place to turn for the latest information on accessibility. Dive Into Accessibility is also a useful tutorial.
3. Follow traditional on-site SEO best practices. You know the drill: major keywords in the title tag, H1’s and body text, keyword-rich anchor text for internal links, etc.
4. Getting spidered and indexed by mobile search engines
If you want mobile search engines to spider and index your site, they must know about it first. Submit your site to major mobile search engines for quick spidering.
Source: Mobile Search Marketing