Nate Beier: step up, jumping to a small box, jumping to a large box.
Sequence for box jumping:
Head forward. Torso upright. Feet just under the shoulders. Dip, drive, contact, extend. Step or jump down from box. Rinse and repeat.
Not dipping the hip before the drive, not landing on target, not reaching extension.
Build your own boxes: http://www.crossfit.com/discus/messages/26/14917.html