I, too, first learned to trackstand on a freewheel. This site proved to be very helpful to me
http://www.teamestrogen.com/content....asa_trackstand
One skill that wasn't mentioned in that article, is being able to pull the bike backwards. To do that you hold a brake (front or rear, choice comes from personal preference), shift your weight back, and as much as possible, prevent the bike from moving forward as well, hence holding a brake. Then with your legs and arms pull the bike towards/underneath you, simultaneously releasing the brake. I find that very helpful when the incline isn't helping you to roll backwards, or if there isn't one.
Once you learn to trackstand on a freewheel, and learn to control the back and forth movement of the bike beneath you, doing the same on a fixed gear almost feels like cheating: It's SOOO much easier!