I have a black piece of electrical tape where the seat post meets the seat tube. It goes around the entire circumference of the seat tube. I had slippage issues until I tried this trick. It seems too easy of a fix and like it would never work, but it sure has for me. For about 1000 miles now. Good luck and let us know if it works for you.

PS - It's also a nice way to mark that perfect seat height.