THEORY & METHOD
At this stage, your horse is probably pretty respectful, trusting, and willing with you on the ground. Unfortunately, not everything she's learned will just automatically transfer over when you are up in the air on her back. Being up above the horse can sometimes even send them into a panic.
A nice transitional period is to do some fence work. It keeps you out of harms way, allows for advancement in your ground work, and lets your horse become accustomed to you being up above her eye.
The concept and execution is simple, do all your basic ground work, but do it while you are standing up on a fence. Start on a lower rung. In the beginning, you'll have to get on and off the fence a bit to help the horse understand (you'll probably also find it easier to balance yourself when you aren't quite so high). Send the horse back and forth in front of you a few times, yielding his hips to change direction. Step up a few rungs on the fence and try it again. See if you can't get your horse in close to you (this has practical applications down the road; if you are at all vertically challenged, like myself, you'll find it far easier to get up on your horse from a fence, especially if they are tall!).
When you can do all your ground work (moving the feet forwards at the walk and trot, yielding the hind-quarters, backing up, etc), you can do a little sacking out. Toss your rope over the horse's back a few times. Rub him all over while you're standing up on the fence. Rub him with your foot, even try draping your foot over his back. You can even go so far as pseudo-mounting your horse. The fence is your safety net; so long as you keep one foot and one arm firmly anchored to the rail, you can simply stand back up if you get into trouble.
Please use caution and common sense. Horses are large animals, with an exceptional amount of power and strength. They are also concerned above all with their own personal safety, and will do whatever they feel it takes to keep themselves from harm. Being individuals that act and react differently, the only certainty you have when working a horse is uncertainty. I am a professional trainer with twenty plus years experience, yet even with the knowledge I possess, I still get hurt from time to time. This blog and the accompanying media are for entertainment purposes only. No responsibility will be assumed for injuries or damages incurred while trying to use these methods at home. Please ride responsibly; protective gear can save your life!