Aerostitch is your friend.
I have these:
http://www.aerostich.com/aerostich-warm-wrap-grips.html ... they work reasonably well, but they're kind've bulky and don't work as well as....
These,
http://www.aerostich.com/heated-grip-kit.html which are cheaper, less obtrusive, and from what I can tell they work better too. (In fact, I plan on putting these on mine this winter to replace the over-wraps, which have ceased working for some reason, and I never really liked the grip feel with them on anyway.)
There are also these:
http://www.aerostich.com/symtec-grip-heaters.html, which I know little about other than they're supposedly higher end...?
No matter what, the grip warmers are a must-do if you're riding in real cold. It's easy to put on another sweater under the gear, or another pair of socks, but keeping your paws warm is, in my experience, the most difficult thing to do. Heated grips or gloves is definitely the way to go.
Good luck!
--Jaeger
