Do a 180 on a BMX

This is really easy to do.

Steps

Learn how to bunny hop.
Start turning once you can bunny hop high.
Ride at a slow comfortable speed and crouch as your about to spring off by doing a bunny hop but as you go to turn.
Bunny hop and turn the way you feel comfortable with and when you are at 90 degrees on your back tire.
Pull up on the rear by curling your toes onto the pedals.
Swing your back end around another 90 degrees.

You need to learn how to fake if you want to roll out smoothly.
Your first try is always your hardest, so you might not be able to do it first time.