Work with your trainer. A flying change is very hard to do, and very hard to make it look effortless. There is ALOT of work that goes into it with both horse and rider. And all horses can be asked differently. My trainer's GP dressage horse, all you have to do is change the distributation of your weight on his back and you get a flying change. So have your trainer work on it with you
I agree, talk to a trainer. But I will tell you how I do it when I'm riding english....
So when I'm going to pick up the right lead I will use my left leg to slide back and push the hip in, and use the opposite rein to tip the nose in. Then ask for the lope. Now that you are loping ont he right lead, slide right leg back to push that hip in and the opposite rein for the nose. I'm a western rider and that has always been the technique I have taught my horses, only with reiners you jut have to change direction and they do it on their own :)
I think you have asked the wrong question. I would ask "how do I tell the horse what lead to come OFF the jump with so I don't have to do a lead change". That is what I teach my students to aim for. Lead changes are a distraction on a jump course and usually unnecessary if the rider is doing their job.
That being said, since I also do dressage I would train my flying changes totally differently that a hunter rider would, so I probably won't go there.
It seems to me like that would be kind of hard considering the fact I only jump about 2'3''. There's not much "hang time" to ask for a new lead in the air, so usually the horse lands on the same one it started on and you ask for the change after.