I installed a pair of Grab On grips. They simply slip over the current grip and add extra width and cushioning which leads to less hand fatigue.
This is very strange. During winter I use removable velcro Oxford heated handgrips, wrapped over existing ones.
I cannot wait to remove them in the summer because they feel awkward and tiring to the hands.
Also, it is very difficult to have good feeling on the brakes and to twist the gas throttle fully.
I can imagine they can be helpful to somebody who has very large palms (I weigh 250 lbs, 5 ft 11 but have unusually small hands).