As was said before, speed is probably you biggest obstacle. If you're going slow, you can start higher on the box. If you're going fast, you'll have to start lower or you'll fly out. Also, make sure you get all the way to 90. It also helps to think of dropping your lead shoulder just a bit and leaning into the curve, especially on a rail, but also on boxes. Other then that, all basics of sliding apply. (Feet should width, knees bent, hands by waist, etc.) Good luck. You'll stick it your next time out...