Best solution (probably available only at shops): use an air compressor with a narrow tip to inject air between the grip and the handlebar at an angle (like spiralling around). This will create an air cushion and you can move the grip around (keep moving the air jet as you apply the air jet, since only in some positions the air cushion is formed).
Less professional method is what I use to do at home:
- Wash bar and grip perfectly, with water and soap, to remove grease (even the grease from your hands). If the grip is new, probably not needed.
- Spill like a small spoon of alcohol inside the grip, and shake it inside a bit to make all the inner side of the grip wet with alcohol.
- Quickly (before the alcohol evaporates), insert the grip, twisting and pulling.
- In case it gets harder from the middle to the end of insertion, two strategies might help:
- Push from the bottom (lateral part) of the grip, so as to "fatten" it in a barrel shape. This increases its inner diameter, allowing it to snap into position.
- Pull from the inner border, like you would pull a sock around your foot, preferrably with both hands.
I am eager to know other alternatives, because this not always work. Specifically, it might take a few days untill the alcohol eventually disapears. Before that, some slight slipping might happen, but usually nothing serious.
Hope this helps.