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I use a dry microfiber cloth. After a minute of rubbing, smears and marks come right off.

Once, I had a smear that wouldn't come off. It seemed to just spread ( probably french fry grease :D ). I used cleaning spray and a cloth from my wife's eye doctor. They were designed to clean her eyeglasses without damaging them, and they worked great on the phone.
 
I either use monitor wipes, or eyeglass lens cleaner with lens wipes. You can get them at any eyeglass store.

What's good about the lens cleaner is, it's meant to clean up facegunk really well without being harsh tot he surface of waht you're cleaning. And, the lens wipes are meant to clean glass without scratching.
 
I like to run it through the spin cycle on delicate. :D

In all seriousness, some decent screen cleaner and a lint free wipe every now and then is all the average user might need. If you work in a dusty/dirty environment, hitting it with some canned air (or any pressurized air would do) might do you good.
 
I'm a germaphobe, so I wipe it down every day with a hand sanitizing wipe. I do have a screen protector over the glass so I don't mess up the screen.
 
My pocket. It does its job cleaning my phone when it is inside and everytime i take it out, it is very clean.
 
My pocket cleans it for me. Phone looks like new every time I take it out of my pocket to use it :)
 
Usually just my jeans. If I want to clean it up well, I use eye-glass solution + a microfiber cloth. Isopropyl works okay if you wipe it off quick enough.
 
I usually use a screen cleaner and a microfiber cloth at the end of the day. However during the day taking it out of my jean pocket usually cleans it up nicely.
 
Lightly moisten a dime sized area near the corner of a clean microfiber cloth, with denatured alcohol. Leaves the phone shiny clean & germ free.
 
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