My fascination with watching girls masturbate basically began when I bought my first wife a vibrator. It wasn't even a serious gift, really. We went into an adult bookstore one day (the only time we ever went into one) and I decided to buy her one of those simple, slim vibes. I didn't think she'd even like the thing. She claimed she had never masturbated, and that she had never orgasmed until I made her come for the first time by rubbing her clit, and later from licking it (which I couldn't get enough of.)
When she got it home, she stripped down and turned it on and put it up to her clit, she immediately went, 'OHHH!!' and I asked her why she said that, and she said, 'oh, it's so good!!' She kept it up against her clit and just laid there with this trance like look on her face, and finally her hips raised way up in the air, her abdomen jerked in rhythm, and she finally came down from the orgasm and told me how good it was. I thought, wow, how much pleasure she got out of such a basically cheap gift.
Anyway, to make a long story short, she quickly became completely addicted to it! I'd be in the living room, and all of a sudden I'd here this 'buzzzz' from the bedroom, and she'd be lying there on the bed with her vibe up against her clit, and her pants around her knees or completely naked. And she got into this basic routine about the whole thing, she'd always lie flat on her back, and place the vibe on the side of her large clit (never anywhere else) and press the other side of her clit with the fingers of her other hand, kind of putting more or less pressure on the vibe that way.
She never put anything inside of her or moved at all, except for the slight movement of her fingers or the vibe. Her nipples would get hard and dark red, and this red rash would progressively spread over her tits, and the rest of her chest. Beads of sweat would form on her, and when her lips started to turn purple, I knew that her orgasm was imminent, she would moan some during the plateau phase, but when her lips started to turn purple, she became completely quiet, and about 15 seconds later, her hips would rise farther and farther into the air, and her abdomen would jerk up and down in rhythm, by this time usually I couldn't take it any more, and I'd make myself come on her stomach or her sexy tits. She was somewhat overweight, which made her tummy jerking up and down during her orgasm even more sexy. She had long blonde hair and her pussy was unshaven, sometimes when she was diddling herself I couldn't resist sucking hard on her nipples, fondling her boobs, but she said she never felt anything except this intense pleasure in her clit, which basically put her in a trance. The interesting thing is that when I would finger her pussy, she would scream loudly when she came, but when she orgasmed using the vibe, she wouldn't make a sound.
If you've never watched your girl masturbate, encourage her to do it for you, it's the hottest thing in the world!