How to make iphone case less slippery?

My iPhone case is really slippery. How can I make it less slippery?

Tree sap.

Sand paper

Don't buy the slippery one in first place i would suggest you to buy rubber case because they are less slippery and they never break and it really protects the phone.

Because of different surface finishment technology, some case surface is slippy, I recommend you buy a new leather case

Why not change a leather one or rubber one such as http://www.onecase.co.uk/element-vapor-comp-patriot-case-for-iphone-4-4s.html or http://www.onecase.co.uk/element-vapor-pro-white-edition-case-for-iphone-4-4s.html