In norcal we've got lots of sand - usually in the form of just fine, loose, dry dirt. In New England, there's some when there was an elementary school or beach involved. We've found the best technique is to stay relaxed in the upper body, and power through it. The worst thing you can do is downshift - otherwise you might as well get off and run before you lose so much momentum you have to shift.
But a new video here makes us question whether that's the right approach. Perhaps neon green animal suits and a girl blowing garlic breath down our noses is a better way to get better at riding sand.
The technique described above is correct, i was also told to keep my weight to the rear of the saddle, try to stay partially upright in the saddle. this was at a free cross clinic. The person instructing called it "motorboating" envision a boat in the water, looks light on the front, heavier in the rear.