I have a Rohloff Speedhub on my HPV StreetMachine and the rubber grip shifter can be hard to turn when I am wearing cold weather glove layers or during the summertime when my hands get sweaty.  To remedy this slippery problem, I bought some stretchy silicone handlebar tape and put a couple wraps on the shifter.  The bar tape wrap does 2 things:  1) The tape is naturally tacky so my grip doesn't slip as much. 2) the tape wraps makes the shifter slightly larger in diameter and easier to grip/turn.  To attach, I just put two wraps around the Rohloff shifter and then cut the tail off at 45-degrees.  Then I tucked the tail in under the last wrap.  The tape stretches enough to allow me to wiggle the tail in under the last wrap.  No adhesive is required, so no stickly gooey stuff around the grip. If I don't like the way it looks or feels in the future, it is easy to remove.  No brake cables or shifter cables to remove and replace.  Just wrap and tuck.
 
The first time, I put too many wraps on the shifter.  It looked like an eight-ball shifter on a hotrod--not really the aesthetic I was hoping for.  It was really easy to get a hold of the shifter, but the significantly larger diameter of the shifter made it difficult to find a comfortable place to rest my hand when I wasn't shifting.  And most of the time, I'm not shifting.