I wouldn't recommend the balls-to-the-stem thing for skipping. For skidding, sure. But skipping is all about stopping fast, and leaning forward will not stop you as fast, as you have less weight on the back wheel. To skip, you want to do a small jump with your rear wheel (think "endo"), lock it, and then weigh it back down. Then repeat (either a full rotation w/ the same foot forward, or a half-rotation with the other foot forward).
To do the endo, lock your leg straight* or almost-straight on one pedal so that the force of the pedal lifts you up off your seat a bit (and forward a bit). At the same time, yank up hard with your other foot. The motion is basically akin to how you skipped as a little kid.
You'll stop faster the less forward your body is and the longer you can hold the skid after your rear wheel reconnects with the pavement (which won't be very long anyway, at least when you're starting out - maybe just a split-second). For me, most of the force is applied with the front foot (pulling up) as opposed to the rear foot (pushing down).
* Disclaimer - locking your knees is traditionally Bad For You. Use your your best judgment.