I'm sure this is old news, but here is what I posted on TT awhile ago;
Best way to reduce it is ride more often. Try some hand squeezers during the week.
What armpump is, is the failure of your old blood cells to leave your forearms fast enough. Therefore the new blood comes in and creates pressure in your arms, which causes the numbness and pain. The pressure then compresses all the nerves and muscles around your blood stream and you lose the proper function.
Now, the best way to prevent it from happening sooner in the race is to get your blood flowing nicely before the race. Jogging for about 20 minutes before your moto, will work. Also, jumping jacks, etc. Anything to get your blood flowing.
Another thing, teach yourself to grip the gastank with your knees so you can loosen your grips on the bars. Good luck!