...if it's a kid's BMX bike, and you want the grip to stay where you put it, use a contact cement between the bar and the grip. It does mean that whenever you want to remove the grip, you'll need to cut it off. But BMX grips get so beat up and most of them are so cheap that this is not a significant disadvantage.

Hair spray does work most of the time, at least on grips that don't get used quite as aggressively as a boy's BMX bike.

Don't follow the instructions for the contact cement by waiting until it's surface dry, or you'll never get the grip to slide all the way on. Clean the bar end with acetone and a rag to get off all the duct tape glue and other junk that's on there now, then apply the contact cement and immediately slide the grip on all the way.

Contact cement comes in liquid and spray forms...either will work. Let it dry overnight.