Consistently big airs in the pipe require a rider to generate and maintain speed throughout a run. While what's done outside (above) the pipe garners the bulk of the attention, it's what happens inside the pipe that counts when it comes to speed.
Give yourself every advantage by entering a pipe from the sides, “dropping in” (see below); this is essential to starting a run with speed. To do this, approach the lip slowly with your board angled slightly down, rather than straight across, the pipe. This effectively lengthens the transition, making the drop more gradual. As you roll over the edge, be sure to weight the front foot.
Carving (with the board only slightly on edge) rather than skidding your board across the flatbottom of the pipe will keep you from losing speed, and pumping the transitions–like pumping your legs on a swing to get it going–will cause you to go even faster.
To pump, you either push against the snow (extending the legs) to increase pressure, or absorb terrain by bending mostly at the knees to decrease pressure. Doing each of these at the right time is what makes someone a pro.
Big Air: How high a rider can jump out of the halfpipe largely depends of the size of the pipe itself. In a competition pipe, with smooth twelve- to fourteen-foot transitions, pro riders consistently reach high marks of eight or more feet. Most really high airs are done on quarterpipes, where the speed of the approach is more or less unlimited. Four- or five-foot airs are a good goal for the average rider in a well-made pipe.
Back-to-back big air out of the pipe is a result of what happens inside the pipe: smooth, subtle edging, pumping the trannies, and good timing.
Dropping in from the side of the pipe is the best way to start a run with speed.