Unless they are some sort of aftermarket grip, there should be a ridge that gets held into the switch housing thereby keeping it where it is supposed to be.
Are you referring to the grip sliding off the nylon sleeve? If so, scuff the sleeve with some forty or eighty grit and slather some RTV on that puppy to keep it glued on.
RTV is amazing stuff. A good adhesive in many cases. I use it to hang mirrors off my windshield. Works better than the stuff made for that purpose. Just give it some tooth to work on!