make sure you serve Googlebot-Mobile the same content as mobile user agents.
Making Websites Mobile Friendly:
however, before you do anything you also want to read and understand the distinctions between mobile phone and smartphone browsers as google sees it.
How does Google treat mobile sitemaps that are almost identical to standard desktop ones? - Webmaster Central Help:
and i assume you are aware of w3c's mobile bp docs.
Mobile Web Best Practices 1.0: