Do you practice "stand." Square her up, position yourself to the side of her head, a couple of feet away and say, "stand." If she moves her head away, shake the rope and say "no, stand." If she moves her head towards you, twirl the rope at her a bit, and say, "no stand." Make her stand for a few minutes with her head in front of her chest, looking straight ahead. If she tries to walk forward, make her back up, then lead her forward again to the same spot. Do this for a few days with no tack, then make her do it with tack before riding. You might make her stand all tacked up for a minute (not too long), then praise her and lead her in a small circle. Repeat, then tell her to stand and mount.
Just some ideas based on Julie Goodnight's Lead Line Leadership.
Or better yet, ignore me, and read JG's q&a on the subject: http://juliegoodnight.com/questionsNew.php?id=90