They sell microfiber cleaning cloths at almost every electronics store in the world. They’re pretty good at removing the smudges, and they won’t scratch the screen.
I found regular tissue (not the ones treated with ‘balm’-stuff) work wonders. And I always have a box somewhere in the house. Just generic brand.
And I’m still a bit of a neat-freak about cleaning it. Somewhere between how I like my glasses (spotless) and my drinking glass (the odd finger print if I just ate chicken wings is OK).
Don’t use paper on glass! It scratches! The iPhone ships with a small black chamois, use that, or get a replacement one from the car-cleaning products section at your local superstore.