When I was in for my 600 mile service at the dealer there was an ol' timer there on his 30 anniversary F800 that had taken a series of large O-Rings and covered both stock grips...not only did it give a very comfortable grip, but it looked nice too. Said he had been doing it for years on all of his BMW's.
My tiny hands probably didn't need it, but if I do end up doing it, I'll post the size of the O-Ring.
2012 BMW R 1200 GS "Rallye Edition"
2010 BMW F650GS • 1989 Honda Transalp • 1989 Honda Hawk NT650 • 1980 Honda Hawk CB400