1: Forearm Pump
Forearm pump is intermittent marked pain in the forearms after a period of exercise or exertion. The pain is due to swelling of the muscles in the forearm. This affects the blood flow to these muscle and causes the oxygen levels to drop. Blood builds up similar to a traffic jam and prevents blood from circulating. Fascia surrounds the muscles and prevents them from blowing up like a balloon. When you stretch the fascia to its limit and fill the muscle with blood the muscle will feel rock hard. When you experience this it will limit use of the hands especially when trying to grip. Not a great thing for us dirt bikers while trying to hang on to a 250lb bike as it bounces through some whoops.
2: Arm Pump Surgery or Fasciotomy
This is a procedure that cuts or removes the fascia around your forearm muscles and allows for more blood to pour into the forearms. Having the fascia cut will prevent them from getting tight and hardened (In theory). This procedure has become very common in the professional motocross world. Some people will do whatever it takes to get an edge. However, be warned that this surgery is far from perfect and comes with risks. In some instances they simply cut the fascia and this can sometimes heal back with scar tissue, doing the opposite of what you intended. In other surgeries they will completely remove the fascia. There is little evidence that this method is good for your long term health or that it will even reduce your forearm pump. I would do some homework before going down this road.
3: Over-training
Over-training can lead to consistent relapses of forearm pump. If you do not allow your body to recover and heal then it can begin building up toxins such as Lactic Acid. When your muscles have a build up of lactic acid they will produce a burning sensation and begin pumping full of blood until you cramp and fatigue.
4: Massage 
Massage your muscles frequently to break up those toxins and knots. Massaging will make a huge impact on your forearms endurance. This needs to become a routine and the best time being before and after riding as well as just before bed. Massaging can also help the muscles recover faster and get you back in the dirt! Go check out this video to see deep tissue massage techniques.
5: Ice
Ice your muscles after rides to help your arms or legs recover from the workout. This tactic is used among athletes across the board and is highly effective. The ice will help reduce the swelling and push the blood out of the muscles. Icing can help reduce the lactic acid build up so that you won’t wake up feeling sore the next couple days. Most people prioritize their forearms because EVERYONE suffers from arm pump.
6: Stretch
Stretch out your muscles thoroughly and as often as you can. If you are serious about racing or just want a long career of trail riding, I highly recommend doing full body stretches daily. Put a lot of focus on your forearms to help combat the arm pump. Staying limber will help prevent injuries dramatically and stretching in the morning will really wake you up and get blood flowing.
7: Bike Setup
Bike Setup is very important for all riders and you shouldn’t take it lightly. You should test different suspension setups, handle bar positioning, brake and clutch lever angles. Learn what feels comfortable for you and don’t be afraid to try something new.
8: Riding Techniques
Riding techniques are extremely important for reducing arm pump. Veteran riders are very relaxed and have acquired techniques for reducing physical effort. Instead they allow the bike to do all the “heavy lifting” and conserve their energy. One of the most important things you can start doing immediately is gripping the bike harder with your legs. This will allow you to loosen your grip and relax a little more. Try to avoid being tense and get comfortable on your bike.
9: Strength Training
Strength training can be a great tool for increasing your muscular endurance, check out my article on how to train for hare scrambles. There is some great knowledge on training methods for motorcycle riders. Check out Ken Roczen putting in work to prepare for the supercross season. The key is to avoid large muscles and train with light weight and high reps to condition the muscles. The larger the muscles the more oxygen they demand which will result in fatigue much faster.
10: Nutrition 
Nutrition is a huge part of any serious athletes routine. Eat healthy and provide your body with the nutrients it needs to give you energy. Nutrition also plays a huge role in how efficiently your body will recover. It’s hard to build a house with limited materials. That’s the way your body feels when it is not receiving a balance of nutrients. You will be amazed at how much more energy you will have. Having more energy will lead to longer training sessions and hopefully a stronger race finish for those of you who compete.
11: Hydration 
Hydration is probably the most important thing of all and your body depends on water to survive. All of the cells in your body require water to function properly. It is used to maintain your bodies temperature, remove waste and lubricate joints. The rule of thumb is to drink 8, 8oz glasses of water each day to stay hydrated. This will vary for different people so figure out what works for you and keep drinking lots of water.
12: Riding 
Riding as often as you can is the best way to condition your body for riding. Nothing else can simulate riding with all of its quirky physical demands like going out and riding. Have days of playing around with friends and have days of being focused on pushing yourself. I don’t mean go as fast as you can and get hurt. Go at a pace within your limits and time yourself. Train for the length of whatever racing or riding you do. I enjoy hare scrambles so riding hard for about 2 hours is what I try to do. You will learn a lot about your fitness level and where you stand currently if you do this type of training. This also helps you learn how to pace yourself and what type of pace you are capable of maintaining.