Just like Carnegie Hall: Practice! Practice! Practice!
On a more realisitc note: Probably the best skill anyone can develop is slow speed handling skills. You should be able to come to a complete stop with both feet on the pedals and still remain upright for at least a second or two. Generally when I come to a stop, I am barely moving and unclipping at the same time. Even after I am disengaged from the pedals, I am not moving as I put my foot down.
Which way do you move your heel to unclip? Most people I see move their heel outward. I've always found this to be a very awkward movement. I prefer to pull my heel inward and I seem to disengage quicker and easier.
When I do fall over (and it happens more often on a mountain bike then a road bike) I try to keep ahold of the bars and present my gluteus maximus to the ground as well as my shoulder and upper back. Try not to throw your hand out to catch you since that's a great way to meet orthopedists
Again, if you have mastered slow speeds, the fall is more of a pride hurter than anything else.
And, finally, as others have told you, lose the pants!
Okay, let rephrase, don't ride in pants! The reason that cyclists ride in tight fitting clothing isn't becuase it shows off our assests
We ride in those goofy black tights and shorts because you don't wanna have lots of stuff flapping around and getting caught in the machinery! It ain't about style - it's function!