Smartphones spend a lot of time in hand, where they can pick up germs and dirt. Wiping down the phone regularly with an antibacterial cloth intended for use with touch screens can help keep it clean. Many office supply stores like Staples or Office Depot carry disposable wipes for use on phone and tablet screens.
If your phone has a camera and your photos have been looking blurry, you can clean its lens with a microfiber cloth or other wipe for use with camera lenses; a cotton swab moistened with distilled water can also take off stubborn grime. Whatever you do, though, do not spray the phone with industrial cleansers or use cleaning wipes designed for household chores, because these can damage the screen and other parts of the handset.