If you're really interested in a school and have done something interesting or worthwhile since you submitted, send an update letter. If there are two schools in the same area, and you have an interview at one but not the other, you could also send an "in the area" email describing when you will be there and reiterating your interest in their school. But this is risky, and you have to write it so you don't come off demanding (there's an example of a good email in the georgetown thread, and that person ended up getting an invite soon after, but again, could also negatively impact you).