Brandon,
Although there are several factors which can contribute to arm pump, I believe the first issue you should address is building core strength. Building core strength in your abs and back will allow your limbs to do less work while you are riding and in return should lessen your chances of getting arm pump.
Good old crunches and back extensions are a great place to start. Make sure you pay strict attention to form while you are doing your exercises. While working your abs, make sure you are using only your abs and not your legs.
Of course there is much more to being fit for motocross than core strength. Please take a look at my website
www.fitnessandfuel.com or contact me at 517.404.9003 if you are interested in taking the next step in your training with a complete fitness and nutrition program!
_________________
Kim Wathen M.S.
Exercise Physiologist/Dietitian/Personal Trainer
kim@fitnessandfuel.com