After removing your old grips and carefully cleaning the bar area, spray a SMALL amount of WD-40 inside the new grip. You may have to muscle it on a little bit, but it will be ready to ride in minutes.

Since WD-40 is water based it will dry up nicely and will not harm the rubber the way petroleum based lubricants can. Technically, since it evaporates and is water based, WD-40 sucks as a long term lubricant.