I can skid on 48x16 in the saddle all day long. Like said before its all about technique. A wet (and preferably freshly paved) parking lot is a good place to get it down.

For me, snapping my 'down push' foot tends to break traction the easiest. Lifting your butt a few inches out of the saddle gives you considerably more stopping power as well. The best advice has been given though already...

Do not go any faster than you can comfortably stop... and throw a muhf'in brake on your bike.