Isopropyl (aka rubbing) alcohol, maybe ever Windex (which has alcohol in it.) Use a generous amount and leave it on for awhile, it might dissolve the residual glue.
Very hot water might work as well as it will melt/soften the glue.
For that matter, you could try heating up the glue with a blow dryer and see if the residual grip will peel off.
As a last resort, you could try WD40. It will dissolve the glue but it will probably take some of the color off the leather grip.
Worst case, buy another grip. No biggie.
It should be a crime to put a replacement grip over a nice leather grip IMHO.
Last edited by Fearsome Forehand; 01-24-2013 at 10:23 PM.