Lennard Zinn in Zinn and the Art of Mountain Bike Maintenance recommends using alcohol. I have used alcohol a few times, especially with a syringe. (No, I'm not a "druggie" but work in health care.) Inject the grip with alcohol and it instantly slides off. Using a screw driver to lift up the grip in order to work in the alcohol also works, but not as “gracefully”. Alcohol also dries very quickly.