Aqua Net. It's cheap and it works good. It not only makes the grips slide when you want them to but, after it sets up overnight, it eliminates the dreaded "throttle grip".
When I install bar ends I move the shifters and brake levers inward about an inch, spray a little Aqua Net under the grip, and pound it inward with a hammer. The hammer and handlebar together make a circular cut in the end of the grip so that you can slide it inward.