I've done this a few times. It's all about trust with women. If she knows that you aren't going to go run off and show your friends the pictures you took, then you're good.
One girl I was with, simply trusted me and would let me snap off pictures anytime I wanted to.
Another girl was simply drunk. I had the camera in my hand. I pulled out her boobie, started sucking, then started snapping. I kept exposing body parts and taking pictures until she was completely naked.
Yet another girl, I had a sit down with her. Told her the things that I wanted to do and had her understand that she could trust me.
So, in the end, it all boils down to trust and alcohol