Hey there!
Sometimes my FP is nearly slipping out of my hand because of the smooth metal back surface.
Ia anyone facing the same problem and may have a solution?
I don’t want to cover my FP with a big rubber/silikone shell.
I found something interesting the other day:
Cheers!
I like this idea 
The guy who did this, even has backings in cork! I think that should help control the slipperiness too, without roughing up your hand as much.
Here’s something else, anti-slip Stickers:
http://flexinno.com/egrips
I know they have a different purpose but I’m guessing if you put them on a phone in a certain way they could also help increase your grip when you hold it in your hands.
Edit: after exploring their website a bit more. They actually do have those Stickers for the purpose of a better grip on the phone.
I use this one, very handy and safe. Good grip. Sticks well to the back of the phone.
Grtz