I don't suggest the WD40 method, but for the last 35 years I have used Gumout, and the red snooter tube, slipped in alongside the grip/bar interface, give it a good squirt and begin twisting and pulling. They always slip right off.
The solvent evaporates rapidly, and leaves no residue.
I re-install using the same method wetting the inside of the grip with a generous spray, and slamming the grip on.
It takes about 10 minutes to become dry inside.