I have a fixed gear bike that I love riding, however I have a tendency to slide forward a bit in the seat. I've adjusted the seat to be all the way forward, but I still slide just forward enough that my sit bones aren't quite on the seat properly. I have a 90mm stem currently and was wondering if I just got a 60mm stem if that would help me to stay on the seat properly? or is it possibly just a bad seat?

Thanks!