You might not believe it if you're just starting out in motorcross racing, but learning to fall is just as important as learning to ride and control your motorcross bike.
First off, when you're losing control of the bike and it's obvious that you're not going to regain control, don't fight it. Quickly assess which way the bike is going to go and literally roll with it. You will save yourself from serious injury on your motorcross bike.
If you are making a turn on your motorcross bike and realize you have leaned too much, swing your leg that is under the bike closest to the ground under and away from the bike, and let go of your bike. Try to roll if you can to absorb some of the impact.
If you lose control of your motorcross bike when you're making a jump, push the bike away from your body and let go. Some people move their hands in a counter clockwise direction in big circles to push some of their body weight away from the bike and slow down a little. This doesn't work any more than flapping your arms allows you to fly, but as long as you push the bike away, you should be safe. Once again, try not to land solid, attempt to roll your way out of a fall whenever you can.
If someone falls in a race, a caution flag will come up on the track. Slow down as soon as possible to avoid hitting anyone or anything. If you have fallen on a track, get yourself out of the way as quickly as possible.
No matter what, if you're in a falling situation, let your bike go. There is no bike worth risking serious injury over. You bike and bike parts can be replaced, body parts take a long time to heal. Make sure you're wearing the latest safety gear and learn to fall, so on those small occasions it happens, you know what to do.