When I was studying in Denmark over the summer, I rented a basement apartment from a family with an 18 year-old daughter. She and I hit it off right away and it wasn't long before we were both pretty hot for each other. One evening in her room, she showed me some (Danish) books and asked me if I knew them. The books were fairly well-known works of Danish erotica, and I told her (truthfully) that I had already read them. She told me that they were her favorite books and that she often got very aroused reading them. I took the bait and asked her 'how aroused' (this was all in Danish, by the way), and she said that she often read them and masturbated before going to bed. I replied that I also had masturbated while reading them, and pretty soon we both had our pants off, taking turns reading aloud from the book and masturbating together.
The longer I stroked, the more intently she stared at my cock and rubbed herself. When I said that I was about to come, she leaned over and took the head of my cock into her hand. I wasn't sure exactly what her plans were, but I kept stroking and a minute later I was pumping my cum into her palm. After I finished, she sat back, spread her legs wide, and finger-fucked herself like crazy with two fingers until she came.
The rest of my stay in Denmark involved almost nightly encounters masturbating together. Most evenings she asked me to cum either on her tits or her ass (but not her pussy, since she wasn't on the Pill and was worried about getting pregnant), or just had me blow my load all over myself. She also put on quite a show for me while I stroked my dick, masturbating in just about every imaginable position (standing, kneeling, doggie-style, etc.).
One thing she particularly liked was having us 'spy' on each other : she would come downstairs while I was taking a shower (I would leave the bathroom door unlocked), and 'secretly' watch me jerk off my soapy cock. She also enjoyed masturbating under the bathtub faucet while I sniffed her panties behind the doorway.
We lost touch after I came back to the States, but I can honestly say that I've never again met a girl who enjoyed mutual masturbation as much as she did, and after all these years I still often picture our encounters when I'm beating my meat. I wouldn't at all be surprised if she were reading this forum as well :)