What is the trick to riding on sand?
Yes there is one main trick, and some regular skills.

The trick is: if you LOWER THE TIRE PRESSURE, any tire will float over sand like magic. It should not be so underinflated as to allow easy pinch flats, but the lower the pressure, the more marked the floating effect. There are just two limitations:


If the tire is skinny (low volume) it might not be possible to have a pressure level that allows flotation AND safe riding over rough terrain on other trail sections;
It's not possible to adjust tire pressure "on the fly" ;o)  But if you know you're having a long stretch of soft terrain, it's possible to have a good trade-off.


Anyway, huge tires (2.3 +) are perfect for that (watch this Surly Pugsley video). Also, knobby doesn't help that much, sometimes slicks are better; they cause less shearing of the sand and rob you of less kinetic energy.



Now the skills are mostly help maintain speed and direction.

Speed: not so fast as to suddenly throw you over the bars in case of loss of control, not so slow that your rear wheel loses traction in order to maintain or recover speed.

Direction: plan ahead the smoothest possible trajectory, and point your front wheel firmly along that trajectory. Avoid wobbling on the pedals, major gear changes, and specially avoid sharp turns, because there is a critical steering angle above which the front tire skids sideways and the bike becomes unstable, making it almost impossible to regain control in some cases (of course the more skill one has, the more one can recover from).

Hope that helps and... PRACTICE! Few things can be more fun than that!