Another thing you can try,that does'nt cost a thing,is....keep a looser grip on the bars than you normally would.A local expert mentioned this to me several years ago,and I will say,it is alot easier said than done,but if you can get past your brain telling you to hold on for dear life,it will help ALOT.
Again,easier said than done.I just got back from a 2 hour trail ride...it rained last night,so there were alot of wet slimey roots,and many times the bars wanted to fly outa my hands,and I found myself getting arm pump/numbness and death gripping after a while.When this happens,I try to just SLOW down and take deep breaths for a few minutes,rather than stopping and resting.If you can RIDE through it,you'll have it happen less and less.But if you get to the point where you feel as though you might not be able to let off the throttle from so much arm pump,by ALL MEANS....SHUT IT DOWN!!