gascanuck,
Check out this link to the woods racer site. Scroll down and look for
drdirtrider's comments. He is a doctor / rider from Washington and has some links with actual exercises you can perform to reduce arm pump.
There are a couple links in there so check them all out, but really how long have you been riding for and how long have you had arm pump? It's pretty common thing if you do not ride all the time or haven't been riding for very long. Anyway good luck with it.
-Josh
http://www.woodsracer.com/portal/for...ead.php?t=8077