The best way to get rid of arm pump is to learn to relax while riding. Gripping the seat with your legs helps also. I rarely get arm pump any more wheather racing harescrambles or on the MX track. Mainly because of what I stated above.
As far as training your forearms goes, I have numerous exercises from my former armwrestling days. A long long time ago I held two state championships and was fifth in the nation in my weight class.
I used to use a variety of exercises for my forearms ranging from very unique arm curls to gloves with rubber bands to strengthen the back of my hands. One thing I discovered is that common hand grips works far more than your hands. Your entire forearm and to a degree your upper arms benefit from this exercise. As you progress, start using two or three fingers at a time and keep working until you can do it with one finger (each finger) at a time. Once that gets too easy (depending on your current hand and forearm strength, it may take awhile) take the retaining clip off of the handgrip and bend the grip backwards. This will spring the handgrips out some and make it wider and more difficult. Know matter what kind of exercise you do, concentrate on endurance (high repetition) not how much weight you are lifting etc.
By far the key is plenty of time on the bike and learn to relax.
Good Luck
[This message has been edited by Monkey Butt (edited 10-15-2000).]