How arm pump happens - the science bit!
During vigorous exercise, muscles require a tremendous amount of oxygen-rich blood and commonly increase in volume by up to 20 percent. The engorged muscle is encased inside the inelastic fascia and, as it grows, the pressure within the fascia compartment increases. Although gases and solids are compressible, fluids are not. The incompressible fluid within the inelastic fascia makes the forearm feel hard as rock.

Most of the studies related to high compartment pressure (what motocrossers know as arm pump) have been documented in the lower legs of distance runners. Only a few cases have been described in the hands, feet, thigh, elbow, and forearm. The condition is called various names, including "chronic compartment syndrome," "effort-related compartment syndrome," "exercise-induced compartment syndrome," or "chronic exertional compartment syndrome." Motocross arm pump is technically known as "chronic compartment syndrome of the forearm," (CCSF). All of the names seek to differentiate this condition from the much more dangerous condition of "acute compartment syndrome" (the kind Larry Brooks had).

Strategy One - Alter Your Riding Style

Altering your riding style has a good chance of reducing arm pump in riders who suffer from the problem. It requires three steps:

Frequent riding. The more you ride, the more efficient your body becomes at delivering blood to the forearms and, more importantly, transporting the waste material out of your forearms.

Staying relaxed on the track. By loosening your stranglehold on the grips, moving your fingers, and alternating between squeezing and relaxing your hands, you can keep blood flowing.

Use your legs. If you grip the bike with your knees, you reduce the amount of upper body strength required to hold on. The muscles of the leg are larger and less likely to pump up than the muscles of the forearm, so shifting the load will help.

Strategy Two - Change your workout

If you train regularly, perhaps changing your training pattern can help reduce arm pump. Realistically, changing your training regime has a medium chance of reducing arm pump.

Avoid heavy weights in arm workouts.

Emphasize wrist curls with light weights and high repetitions.

Do lots of forearm and wrist stretching.

Use aspirin as a blood thinner. The blood thinning attributes of aspirin work best in low doses. One 75mg  pill a day is all you need.

While studies have not proven that proper hydration can reduce arm pump, it is logical to assume that being dehydrated could lead to or increase the chance of pumping up. Drink plenty of water before and after every event. Some pro’s claims they drink at least a gallon a day!

Strategy Three - The Scattershot Approach

Arm pump can come from a variety of different sources. Although your chances of eliminating arm pump with these tips is slim, you never know. Here are areas of concern for any racer:

Try taking nutritional supplements, vitamins, magnesium, potassium and calcium. These can help alleviate chemical imbalances.

Change your bar bend, composition (aluminium) or position (some say up, some say down).

Alter your lever position (some say up, some say down).

Try different grip sizes and densities (some say smaller and stiffer grips helped, while others say a larger and softer grip absorbs vibration better).

Steering dampers can lessen your death grip.

Gripper seat covers allow you to maintain your position on the bike with less arm strength.

Acupuncture, magnetic therapy and crystals could work on a psychological level.

If the "compartment pressure" rises high enough, blood vessels can collapse, which restricts or stops the flow through that vessel. Veins, with their low pressure and thin walls, collapse earlier than high-pressure, thick-walled arteries. When veinous flow reduces, arterial blood continues to enter the fascial compartment but is restricted from leaving. This restricted outflow further increases the pressure within the fascia compartment. If the compartment pressure rises higher than the pressure in the capillaries, or even the arteries, then these vessels may collapse, resulting in "muscle ischemia"-a painful condition of oxygen deprivation. Muscle ischemia leads to even higher compartment pressures.

Arm Pump