The way I learned to skid was, I was slowing down by pulling up on the front pedal and pushing down on the back pedal... and then one time I did it hard enough and I skidded.

I rely on the leg pulling up on the front pedal to lift the rear wheel--don't really lean forward much. In traffic, I do little skids, one side then the other, in quick succession. If I need more stopping power I fishtail and drag the wheel sideways. (I can only fishtail to the right though, go figure...)