I bought 2 pr of these:
http://www.amazon.com/NOJ-Comfort-Grip-Wraps-210/dp/B000WP9KUE
and they work very well for me.
They're available in many places, eg.
https://www.denniskirk.com/black-neoprene-comfort-grip-wrap.p501629.prd
•Decrease vibration and increase comfort
•No need to replace your grips for an improvement
•Hook-and-loop system fastens quickly and easily while minimally increasing grip size
•Fits most motorcycles snowmobiles and personal watercraft
•Machine washable for extended use - 4 3/4 in. long
•Fit 7/8 in. or 1 in handlebar grips with twist throttle
They are very easy to attach as they simply wrap around the grip and velcro together, just like fork 'Dirt Skins'. Adjusting the seam on the grip is key for comfort.
FYI, these are fairly thick(4-5mm), and so someone w/ a small hand may find the grip then Too big for comfort, as is the case for my 17yo son. This could also be an issue w/ the non-wrap/slip-on type.
If thickness is an issue, someone could also buy thinner neoprene(that which comes w/ hook-and-loop style backing) say 2-3mm thick and so fasten w/ velcro(hook and loop) fastener.
hth, dean