Just wanted to rant about this being the slipperiest phone I've ever had. Just dropped outside a training class for work. Thankfully I was using Spigen Slim Armor that cushioned the shock and only ended up with a knick on the case itself. I can literally grip this phone and have it slip through my hands.
1st: Check your local supermarket. There are a lot of nice grease regulator sops available! they are nice for your hands making it soft and with good grip (This was only a joke!!)
2nd and now the serious part there are some pretty good back covers gives the phone a nice grip feelling
here are the two I bought
http://www.ebay.co.uk/itm/3710932024...84.m1497.l2649 gives you a nice sandblast type of feeling similar to the one you get on the one plus one!!
http://www.ebay.co.uk/itm/1911997899...84.m1497.l2649 this a more plain hybrid case but also with good grip
Both are not finger print magnets as most of these cases are