Create a mobile version of your website. This isn't an app, but it will come in handy when you build one. A mobile site will give you a presence on all smartphones, regardless of operating system, and provide valuable insights into how people interact with your brand and content on a mobile device.
Think small. Just as phones are not computers, apps are not websites. The best apps are elegant and easy to use, and perform one or two core functions that make users' lives simpler, better, or more fun. Remember: Smartphone apps are designed to perform on screens no bigger than the palm of your hand.
Decide on one or two key features to include in your app. "Companies have far more ideas than can possibly fit well in the context of a single app," says Raven Zachary of app developer Small Society. "Successful apps tend to have a real core function."
Browse the various app stores for similar apps and read the user reviews. Apple's iTunes and BlackBerry's App World webpage make browsing and searching apps by keywords rather easy. Google's Android Market webpage, however, does not include a search function (though one is in the works). AndroLib.com and AppBrain.com can be used until Google improves its app webpage.