Press/crouch down, kinda like a cat gathering itself for a leap, with arms and legs. Pessing down quickly will peload/squash the tire and if you leap quickly enough to use the rebound of the tire and follow through with a contraction you'll gain more "spring", timing is important. As the leap leaves the ground pull up and contract you limbs. Pulling the bike up with you will give you more height, because of the obvious and it helps your upward momentum. Having a little speed helps the stability, (more gyro-scopic effect from the wheels). Practicing on grass may be a good idea, easier landings when you make the inevitable mistakes. Using a "trashmo" bike may be wise as well.
The cycling magazines, at this time of year, will often have articles with pictures to explain the techniqiue. ie- VeloNews, Bicycling, Mountain Biking, Cycle Sport etc.