STYLETHREAD -- LET'S TALK SHOP!

I get free massages from a girl who is going to become a massage therapist.  She finished school but needs a certain amount of hours on clients before she can get officially certified, so she does free work.  She still accepts tips, and since she goes slightly over an hour, I give her 10 bucks.  She does more work than a regular massage; its called "trigger point" which basically means she finds the really bad knots in my back and does a lot of pressure and after enough sessions, the knots will loosen up and/or release. 

Then I went back to college, and now that I'm back for a month, I called her and she said she can either do a free deep Swedish massage, or I can pay for the more advanced trigger point techniques she had learned while i was away (which would be the most helpful for my back knot problem).  She charges for that type of massage because it is harder work. I am choosing to pay and get the more beneficial trigger point massage. Here is the problem.  She is only charging because she needs cash, so she is cutting me a deal (so these arent fixed prices).  She said its 25 dollars per session (session = slightly over an hour), or, if we make regular appointments, I can do twice a week for only 30 bucks a week (which means only 15 bucks a session).

I chose to do the twice a week option.  So do I tip at all since she is charging and the money for the actual massage goes right in her pocket? And if i do tip, how much should I give her? Also, I pay the 30 dollars at the second session of each week, so if I tipped, would I tip only at the second session when I paid for that week, or do I tip at the end of each session?

Sorry this is so long, but being a good tipper is really important to me and she is giving me a really good deal, so I want to do what is best! Thanks!