Seems like one question I get asked a lot is: "What is the best way to care for a slate friction call?" I would say the number one thing I try not to do is touch the slate with my fingers. The oil from your hands and fingers can leave residue which can make a dead spot on the slate. I prefer using scotch-brite instead of sand paper to roughen up the surface. Just a small piece works good and I keep it in a Ziploc bag with my slate call. This can be used to roughen up the slate and also on the bottom of the striker. I normally keep my calls in a Ziploc bag in my vest just in case you get caught up in bad weather while out turkey hunting. 
This just seems like an easy way for me to keep my calls dry, and a good way to store them during the off season.
This just seems like an easy way for me to keep my calls dry, and a good way to store them during the off season.