It's just one of those inevitable facts of life if you drive anywhere other than paved roads; eventually you're going to find yourself in a position where the tires lose grip and your car or truck refuses to move. Driving on sand -- particularly the soft "sugar sand" found in the American South and West -- is like trying to walk on a pit of marbles. Sooner or later you're going to sink in right up to your lug nuts.
Roll up your sleeves and dig the sand out by hand either in front of or behind all four of your tires. Which way you go depends on several factors, including how much room you have and the terrain. If you're on a hill, dig the sand out on the downhill side of the vehicle. Dig all the way down to the bottom of the tire and dig a 3-foot-long channel in the direction you choose.
Remove your car's front floor mats and flip them over so that the rubber side faces up. Grab the short side of the mat and shove it up under the tire. Alternatively, you could use branches, plywood, aluminium plate or your grandmother's favourite Persian rug; the idea is just to get something under the tire that isn't sand.
Remove the caps from the valve stems on your wheels. Flip your air pressure gauge over to the side with the metal nub sticking out of the middle and press it into your valve stems to release half of the air pressure on all four tires. Half of the air pressure on a standard 35 psi tire should be 17 to 18 psi, but don't go below 12 to 14 or you'll risk unseating the tire. The tire will flatten out a little and drop onto your floor mat or removal material. They will also tend to "float" on the sand, rather than digging in.
Start the engine and put the vehicle into either reverse or its lowest gear, depending upon the direction you're going. Have two or three strong assistants push on either the hood or the trunk lid. On the count of three, have them shove while you floor the gas. This isn't a time to be ginger with the throttle; getting out of mud and sand demands wheel speed, horsepower and momentum, not low-speed torque.
Have your assistants keep pushing until your car is free and they fall flat on their faces. If your removal skids don't work, remove them and keep digging the sand out from under your tires. If you're sunk up to the axles then you may need to lay on your stomach and start digging the sand out from under your car's body.
Drive slowly to the closest petrol station and refill your tires, or inflate your tires with your portable compressor as soon as you get back onto pavement. Get to a hose or car wash and wash off the underside of the car ASAP. Sand gets into everything, and allowing it to sit there and grind away at your chassis and drivetrain isn't going to enhance the car's longevity any.
If you intend to make a habit out of getting stuck in the sand and mud, then you might want to invest in a 12-volt air compressor to keep in your trunk. Dropping air pressure is fine if you're somewhere close to a petrol station, but driving on underinflated tires on asphalt is a sure way to destroy them. Emergency air compressors are cheap and small enough these days that no off-roader should be caught without one. If all else fails, you could always squeeze some cardio in and use a bicycle air pump. You also might want to invest in a set of Maxtrax removal skids. Maxtrax are 3-foot-long plastic skids designed specifically for this task. They're stackable like Legos, and even feature a shovel-like indentation on one end so you don't ruin your manicure.