Black Electrical tape, like the duct tape or the golf club grip, where you wrap a couple of layers around the handlebar and then work the grip over the top of itis what I used on my left ISO grip and it is solid.

 

I have a Heritage Softail and my left grip came loose and would twist in my hand. It was dangerous when manuvering the bike in parking lots and just pushing it into the garage or backing in while parking. I just didn't feel like I had a good handle on the bike. I glued it on and it is solid now. I hope I don't have to take it off anytime soon.
Tnicean

 

Tried the hairspray thing and it didn't work for me. I thought it just wasn't dry, but after a week I realized it wasn't going to dry.

I cleanedthe grip and barsoff real good andjust pushedthe grip on dry. Worked like a charm... but my grips are all rubber and fit the bar really tight, others may be different.