Google AdWords is a great source that allows you to filter by mobile keywords; it provides hundreds of keywords you couldn't have ever thought of.  If you are optimizing for several devices be sure to search for specific keywords since each device is built differently.  If you want to maximize SEO for mobile content, build a separate mobile sitemap from the main site so that search engine crawlers can identify the structure.  SEO and the effectiveness of your mobile content is important. 

By now, you might be using Google Analytics as a tracking tool but consider hiring SEO experts when targeting several mobile markets.  Mobile SEO servicers adhere strictly to website accessibility guidelines and can ensure you get a return on your investment.  If you haven't done your research or if you're not familiar with these guidelines, let the experts handle it.  Never take on something you can't handle.  You should know your customers, so target their needs by using keywords and making it simple through mobile devices.  Mobile users want fixed information.  Always make it easy accessible and keep your content up to date.