use a pole on the ground to do hanged over to get the idea of changing lead and into another direction.
I have found that working on a figure 8 pattern works very well. I start out cantering one direction, turning into the middle, halting, switching leg position, and picking up opposite lead canter from the halt works very well for my horse. It helps her understand the concept a little better. I also over exaggerate my leg position to start out with (keeping my inside leg at the girth and outside leg behind the girth, halting, switching both at the same time, and asking for the opposite lead). Hailey Avery's method of using a pole is also a very good method. I have used it a lot and found it extremely helpful as well.
Poles set straight on a diagonal and ask over the pole, simple changes really exaggerating the change of bend and cues you would use to ask for a change. Make sure your experienced enough or have a coach though because changes can turn out pretty bad if you don't teach correctly
A few tips:
•the more muscle they have through their hind end and back, the easier it'll be for them to make the change, so get them punching forward to your hand and underneath themselves so that they can strengthen their muscles and really get them working.
•a crooked horse will not do a change. The straighter the horse is the better, so focus a lot on bending and straightening. A good way to do this is through circles and cerpintines.
•work a lot with changes of bend and getting them to come back to you. This will help when you initially ask for the change of bend, then to move their shoulder and swap leads.
•start with some help from a pole. Most horses find it much easier to do flyings over poles, as they require them to lift their feet more and use more of their body.
Remember to strive for baby steps, with the ultimate goal in mind. Best of luck!