Riding a dirt bike in a deep sandy dry wash can be a large and legitimate fear of many new riders. Stay in a relaxed riding position, but remain focused since sandy terrain is always changing. Look and think ahead to predict your body placement over the bike to make good decisions how much throttle and brake to use. Work on gettin used to riding in a neutral body position in the sand. Standing up on the foot pegs lowers your center of gravity. The bike can flow underneath you while maintaining control of the levers, pedals and throttle. The bike can be bumping, wiggling, diving and swapping, but footing allows all this to happen and not affect your upper body and control. Use foot peg pressure to lean your bike left or right permitting direction and line change. Carry your body weight in your largest muscles. Your legs absorb the bumps with your lower body. Grip the bike with your knees and ankles in straight-aways. Make an effort not to fatigue your upper body. "When you are riding a sandy motocross track, the key to going fast is to flow through all of the corners and tricky sections. Sand sucks up all of your forward momentum, so using a cut-and-thrust riding style just doesn't work". -John Dowd (pro MX champion)
As you approach a sandy turn keep the throttle on a bit and slow yourself with slight pressure on the rear brake. This causes you to slow but keeps the front end of the bike up so it doesn't plow. Use your front brake lightly, if at all. As you sit for deep sand corners stay in the middle of the seat, not close to the fuel tank. Try not to weight the front end of the bike like you usually do in harder dirt packed corners. Once you slow down it's hard to get your speed up. Keep your front wheel up, loose and light on top of the sand. Keep a finger covering the clutch lever. Be ready to slip the clutch in case the motors lower revs don't respond to exit a turn with enough power.
"Yeah, I learned that the hard way by sitting up by the tank. The bike did the death wobble and the next thing I knew I was picking up pieces of me all over the trail". -Paul F. (novice rider)
Keep your weight more rearward in sand. Lean rearward to load the rear wheel and get traction. It's like driving a boat in water. Keep the bow up and remember how forward momentum stops real fast once you let off the gas. Sand washes are a lot of fun to ride and necessary to learn if you ride California deserts. Practice and build up your confidence and "Fear No Sandwash"