Jump squats. Assume your fighting position and jump in the hair as high as you can, switch stance while in the air, come down onto the balls of you feet and squat down really low and repeat.
Do knee ups but do them while jumping. Jump up and pull your knees to your chest. come down, bounce once and repeat.
Do some resistance training with your legs if you have any equipment for it. If not then build up your speeed. Do lunges, sprints repeately, etc...
It also helps to be light weight. My friend weighs 130 lbs with no fat and he can fly 6-7 feet in the air. So burn off your body fat.
Last thing you will need is some distance to run to get your momentum going but remember you dont want to go flying through the board (only 2 inches is need). You have to be able to break it and land in a good fighting stance w/o crashing into your holder(s).