the best thing i ever found for fitting grips and keeping them on is black tea with sugar in, pour some in the inside of the grip before fitting , when it dries the sugar acts as a glue, job done!
posted this link before but its quite funny and informative in some weird way (see the bottom section on grips, yes I know its a BMX but the same principles apply) http://www.gsportbmx.com/tech/handlebars.php
Some of the other articles in there are relavent to MTB's as well, george has a great way of explaining things in plain english.
It also subjects you to less ridicule from girlfriend/friends. Having Baby Talc for inner tubes and Hairspray for grips is somehow not a convincing excuse for non-bike types
it has more than one use for me , it is a good pick me up at the end of a rebuild and it dosent waste the dregs at the bottom of the cup and usually contains the most sugar where i have not stired it properly.