1. Its simple, go to Circuit City, buy some small cameras to plant around the room and start filming.
2. In a few days, get her to sign a consent and release form, but tell her its your life insurance policy that you're making her the beneficiary on and you need to submit it to your work for approval.
3. PROFIT.
And theres an added bonus, when she goes to kill you for selling your private sessions on the internet, she gets fucked again when she discovers there is no insurance policy. Shes your girlfriend, so she'll understand why you hate her so much.