I use chapstick on all of my Grysons. My new ones are all super stiff but My two oldest ones (about 2 years old) glide easily now. Turn on a good sitcom, plop yourself on the couch, slicker up the teeth with clear unscented chapstick and have a zipper-fest for a good half hour. They should work good after that...and your arm muscles get a work out.
Thanks to Lexie, I tried the chapstick on my Skye's zipper. It did help. All the while I was dragging that zipper back and forth though, I was worrying about whether or not the chapstick would stain the leather if it accidentally made contact with the body of the bag. Based on what happened to a load of my laundry when a tube of chapstick went through the dryer, I would be very careful to keep your eyes on the bag rather than the sitcom while you do this!