I use contact cement. I apply a little inside the grip, let it dry. Then, I moisten the inside of the grip with a few drops of lacquer thinner or acetone and slip it on. If the grip is not tight enough on the handlebar, I wrap a little electrical tape on it.
Have a bike in the shop right now that the owner used Gorilla Glue. Big mistake. That stuff swells as it cures. It caused the throttle to tighten on the handlebar, and now the throttle won't return on its own. The throttle AND the grill will have to be replaced, because the Gorilla Glue is permanent. With contact cement, you can just dribble a little lacquer thinner in there, it will soften, and you can slide the grip back off.