I think the more concerned you are with stepping on your partner's feet, the more hesitantly you'll move and lead, and the more you will actually step on her. If, however, you move confidently and lead from your chest (don't just go sticking a foot out there and then step onto it) it will actually be harder to step on her. (Caveat: she's got to know how to move in response, too.) Also, think of walking "through" her, not "around" her. I know it sounds counter-intuitive, but it works.