I was taught that when climbing a loose surface, one should-
a) shift to a higher (i.e., harder) gear, and
b) assume the same position as a jockey riding a horse - your butt should be hovering slightly above and behind the saddle, your torso should be bent down.

The higher gear is required because you'll be using your full body weight on the pedals so you can power up the hill (rather than spin up the hill). The "jockey position" allows you to dynamically shift your weight enough to the rear to maintain traction if you feel the rear tire starting to spin out.