You might look into John Lyonn's "Go Forward" lesson. It works great in a roundpen, but you can also teach it on the lead. It is a great way to get her moving her body, and building that conditioned response system. If she isn't already great with ground manners (as it sounds you have done a pretty good job) this will improve on them!
I'll give you an idea of what I'm talking about. First you will want to start on a short lead, say.. your hand just a few inches from her face. You want to stand on her left side, about a foot and a half away from her and right behind her elbow. That way you aren't impeeding movement. You will want a whip that is long enough to reach her hip. So you are in position.. left hand grasping the lead and your right holding the whip. You want to think "move forward" and start tapping her butt/hip with your whip. You wan't it to be an irritating bug type tap. Not Wack, Wack, Wack. Just Tap, Tap, Tap. You will keep tapping until she takes a step forward. Some horses take forever to grasp this, while some get it imiediately. Its important that you don't stop tapping until she has moved forward. Don't stop if she goes side ways or backwards or any other direction but forward. At the exact momment she moves a foot forward, STOP tapping. Praise her! Then start tapping again. Once she can walk forward consistantly on a short lead, make it longer. Eventually you will have a horse who can walk on a long line around you for hours!
Now obviously you have to be able to stop her, Hah! So lets say she is moving forward and this time she doesn't stop on her own.(the first few times she moves forward, she will probably stop pretty fast.) You want to take her head and gently pull towards her hip. Don't release that pressure until she stops. It also helps if you walk toward her hip. So technically you are disengaging(I know that is spelled wrong, spell check isn't working D:) the hip and stopping forward motion.
Well I hope that was helpful!!