A metal coat hanger cut to form a crank shaft works great. Its basically an 8 gauge wire (no flat spot to damage the throttle grip like a screw driver). You slip it in, and start cranking it and winds its way around the grip. Add some detergent and give it a pull.
I recommend cleaning the grips inside to ensure the mold release agent is removed so they stay put.
I sprayed some semi gloss spray paint into the grip and slid it on. In 20 minutes it seemed strong and left on a ride. It never moved.
I've always just cut the old grips off, but for putting the new ones on I tried windex last time and it worked great, slipped right on and the ammonia dried it right up and stuck like glue.