Cyclops has the right idea for preventing numbness and cold fingers, without any cost...just simply do not GRIP too tightly...in fact I "hook" my fingers around the grips to just contact my thumb. Try this the next time you have cold hands, ease the tension around the grip and if you "were" holding on too tight, you will feel a rush of warmth as blood circulation gets to the inside of the hands, this area should not be cold anyway as it is out of the airflow.
Don't feel bad, gripping too hard is a common "habit"...racers learn this trick so they have better "road feel" with improved throttle and control response. The great Eddie Lawson was asked why he did not have any wrist and carpel tunnel hand problems after multiple heat race weekends after 10 years into his MotoGP World championship campaign...he does lots of physical conditioning, but simple hand/wrist stretching exercises before the race by pressing the hands back with the opposite hand..."pancake flexing"! Try it...it works!