Snowplow
Easiest to learn on alpine skis. Impossible on a snowboard. Most useful with touring skis.
To Snowplow make a upside down V pointed away from you with your skis. Simultaneously do the following.
  1. Push your skis outward with your heels.
  2. Tip the skis on their inside edges.
  3. Hold the skis in a V position.

Of course you have to get going first before you slow down or stop. Once you get going slide the tails of your skis out to the snowplow position.

You should have your knees bent to absorb shock and have your hands low. This is too learn at once, so first practice sliding down hill with your knees bent and your hands low and forward.