I had many sessions throughout 2010 on various parts of my body and I didn't tip once; and I must say for what I'm paying, it never occured to me that I should be. Then in November I noticed little "gratuity" envelopes appeared at reception as I was making my next appointment. So at the end of my last treatment before Christmas, I left a card containing a token of my appreciation in it, on the counter after I had finished dressing. My tech called me an hour or so later to thank me and said it was "the best gift ever!" I told her that I appreciate what she is doing/has done for me. I'm happy with my results and she always makes me feel comfortable in a potentially awkward situation. I'll probably give her another "thank you" when I reach the end of my initial treatments.
That said, if you don't feel that a monetary tip is necessary, a simple thank you card or note sans cash will still go a long way; it's more intimate and meaningful than a "thanks" as you pull your pants up. I think we could all agree that what these people are doing for us is often life changing.
I copied and pasted my reply to a similar post/question as well.