Well, stand off of her shoulder, make sure that the whip if you are using one stays behind her and use the voice commands along with it. Praise her as soon as she does what you want her to. If she starts to come to you, call her to you, then pat her and start all over.
Just be very consistant, firm, and patient. By the way, don't work her too hard as long as she's nursing a foal.
I think our advice here is in conflict. I wouldn't allow or call her in if she starts to come in. ;~D