How do I keep a female subordinate from casual physical contact that makes me feel uncomfortable?

There is a lovely female subordinate at work who is gushing her thanks for relatively small things that I do to help her. She also frequently either gets my attention or re-enforces her gratitude by gently putting her hand on my back or squeezing my shoulder.
I think she does this because she has learned that this form of communication with guys gets positive results. I do not think that she is being coquettish or a tease or that she is into me. I think it is just a communication style that has yielded her results.
The problem is that it makes me feel uncomfortable. I really have a lot of neuroses when dealing with women. I do not want to bring this up verbally because really I feel she is doing nothing wrong, so I don’t want to make her feel bad or weird. But maybe something in my body language to discourage her from doing it any more.
Thanks.