hi
first post, probably will be on here more often, im from brisbane, australia, into collecting ratty old bikes and bike parts and maken em into cool .....
the way i deal with this is 2 steps
1. squirt a bit of bike chain oil and in between the grip and the handlebar...
2. use a vice grip to grip around the grip with just enough tightness not to tear the .... out of the grip when you twist, but enoguh to be able to move it without it losing traction and tearing the .... out of the grip by lack of grip. this method is brutal and can ruin grips but works REAL well if you're careful.
twist until you break contact between grip and handlebar and it moves freely on the bar... then do it again at another position on the grip... or 2... or 3 or whatever, just twist the grip so its not attached to the handlebar anymore with the glue or melted on or however it got so stuck.
then simply pull it off, you should be able to just jimmy it off by hand then.
with ...... grips i don't plan to keep, i just carve it in half with a knife and peel it off takes 3-5 seconds with a sharp knife.