Keeping your iPhone clean is an important part of keeping it in proper working order. Additionally, a clean screen keeps your iPhone looking crisp and new, and it will make reading the screen much easier. There are a number of precautions you can take to avoid smudging the screen with fingerprints.
Wash your hands regularly, especially before you use your iPhone. Smudges are caused by oils secreted by your fingers -- not necessarily dirt.
Wipe your fingers with a dry cloth before you touch the iPhone screen. If you touch the screen with wet fingers, the touchscreen will not work properly.
Use a fast-drying hand sanitiser before you touch the screen to eradicate oils and grime.
Purchase and apply a fingerprint-resistant film to your iPhone screen. Many brands offer films that protect against scratches and water damage in addition to fingerprint smudges. To install these types of films, lay the film down on one end of the touchscreen. Gently roll the film out across the screen and press to remove any air bubbles as they appear.
The iPhone's screen is oleophobic, so it is already resistant to fingerprints.
Never use window cleaners on your iPhone screen. Instead, use a dry microfiber cloth.