Physically
Before you go to a race you must first be a physical shape. This means that you should be able to at least complete the required laps without slowing down or getting lazy. By doing that you must be exercising and working out almost everyday. This doesn't mean it has to be intense training, but it does require some effort. A good workout would be going for a long or fast bike ride that requires more strength and gets your heart pumping faster. Swimming is also great because it works every muscle in the body. Running helps a lot too if you have the time to do that. And of course, there is always riding your dirt bike that will help.The more you ride that the longer you should be able to last during a moto. Practicing in the heat can be difficult, but it will be an advantage in those longer motos when the heat starts getting to the riders. Working out is good, but if you do it too much to the point that you are sore for more than a day after doing so and continue to work-out, then it does not help you. In fact, it's bad for you because you don't let your muscles heal from working before. So I suggest that you find a good routine that gets you working harder, such as going to a gym if you have the time, running, swimming, or riding your bike. Rest is also a must, so be sure to get plenty of Sleep in Your Bed before a Motocross Race.