Well, I can't say that I have officially traded services under some kind of ongoing agreement... but I, as a student, have sometimes done things in exchange for individual lessons. The most common thing has been turning pages in exchange for a lesson or coaching.
I thought a bit about what kinds of things I would accept from a student (or student's family) as payment in exchange for lessons with me. At one point, I offered to teach some voice lessons in exchange for some painting lessons... so far nothing has materialized though from either one of us (busy schedules). Also, I used to teach a somewhat major actor in my area and have thought about doing some kind of exchange with him... piano lessons in exchange for acting lessons. There are other things that I feel I could use help with around here... sometimes I feel like I need an assistant for my business... hee hee. But, anyway, the point is that these are all things I really feel would be of interest to me in the first place, and they are things that would be an ongoing exchange. So, if what I was needing in return for teaching wasn't happening, I would not keep teaching.
Your situation sounds a little frustrating. I will admit that I have often wanted to be "nice" about things and have had a difficult time in the past getting straight forward with students, but in your case, I would certainly consider letting your student know that this is not okay what is happening. That you need payment of some sort or else your lessons with her will need to stop.
Now, as far as "never trading again"... that is obviously a personal decision. But, remember, this is an individual and all people are different. Perhaps another option rather than deciding that you will never trade again, would be to decide that *if* you do ever trade again, the agreement is very plain and clear. That you are quite a bit more strict, and if it is not happening as agreed upon, you know within yourself that you won't let it continue happening to the point of frustration for yourself.
So, in summation

:
1. Have it be something you are independently interested in the first place (not just their idea, but something you have been looking for having a way to have be part of your life).
2. Make sure the agreement is clear, and if not going as agreed upon, terminate the lessons before it gets out of control.
Hope that helps

m1469