You probably already keep your phone clean. Maybe you use your sleeve to wipe the touchscreen at least a dozen times a day. Of course, better than your sleeve is something called a microfiber cloth. This item can be found at any computer- or office-supply store.
Never use ammonia or alcohol to clean the touchscreen. These substances damage the phone. Use only a cleaning solution specifically designed for touchscreens.
If the screen continually gets dirty, consider adding a screen protector. This specially designed cover prevents the screen from getting scratched or dirty while still allowing you to interact with the touchscreen. Ensure that the screen protector you obtain is intended for use with your specific phone.
You can also find customized cell phone cases, belt clips, and protectors, which can help keep the phone looking spiffy. Be aware that these items are mostly for decorative or fashion purposes and don’t even prevent serious damage should you drop the phone.