I've never really thought out a written explanation, but here is what I do...I think.
Skid in a straight line and get the comfort spot over the bars where you feel like you have the best control.
Start turning towards your 180 direction of preference. (Think long swooping skid like going around a curve)
When you feel like you are at a comfortable speed to do your 180 use your hips to push the bike around and twist with your upper body/shoulders at the same time.

Then, go back in the opposite way to pick up your hat that fell off 20 yards back and do the same thing but this time lean down and pick up your hat mid 180 and then you're set!