Mm...one thing we do sometimes is set up a ground pole in the middle of the figure 8, or serpentine, or wherever you are going to initiate the change. Then you ask for the change just as the horse is going over the pole. Ideally, you shouldn't have to use this...but the world is not always so ideal:P Just keep in mind that the horse just may not be quite ready for flying changes, you may just need to be patient and work on the building blocks more, not just simple changes.
I found a short description of the technique online here: http://www.mindspring.com/~jonknop/horse/039704.htm
Quoted from the link:
"If you are having difficulty teaching your horse the flying change of lead, place a pole on the ground twelve feet from the arena railing, lying parallel to the railing. Canter your horse toward the railing and, as the horse takes off to hop the pole, ask for the flying change. The pole will give the horse a little extra elevation, which makes it easier for the animal to do the change, and the barrier of the railing will encourage the horse to make a clean change--that is, to change both in front and behind. Ride a figure eight pattern, asking for the change each time your reach the pole. After doing this exercise several times each day for about a week, try taking the pole away and performing the figure eight, asking for the changes at the place the pole used to be. Be sure you are not throwing the weight of your upper body toward each desired lead (hehe, this was MY problem when I was learning the aids)
, but are properly using the aids of an indirect rein and outside leg--that is, for the right lead, use a right indirect rein with your left leg applied in "behind-the-girth" position, which is about four inches farther back than your leg would normally be. For the left lead, the aids are reversed--left indirect rein with the right leg in behind-the-girth position.
Also, be sure to stay with the motion of the horse as it jumps the pole, rather than letting your seat pound onto the horse's back. By tilting forward slightly on your crotch as the horse crosses the pole, you'll prevent being left behind."