I'm thinking about relating more of my experiences, but thought this may be of interest as starters.
I think I started masturbating, or at least trying to, when I was around 13 or so. I know that by the time I was in my mid 14s I was doing it pretty regularly. I remember, because that summer my parents were having work done on the house and I had to share a bedroom with my sister. Her bed was nearest the window and as it was a large house set in the country with no near neighbours, we would often sleep with the curtains and windows open.
I soon learned that moonlit nights allowed sufficient light for me to see and I would often feign sleep to watch her masturbate. She'd often glance in my direction to check I was asleep, but she never really caught me looking... lol. She'd bend and part her knees, making a tent over herself while she did it. Sometimes the duvet would fall off her shoulders and I'd be able to see her other hand playing with her nipples.
She made a good job of controlling herself during orgasm, but very occasionally when they must have been intense, she really struggled to remain quiet and not move around too much. I would control myself as best I could and would wait until she'd fallen to sleep, which she usually did pretty quickly after cumming. I'd then slip one hand under me and bring myself off.
There was one notable occasion in this time too. She had started dating a local boy and our parents would let her stay out later on Friday and Saturday. This one night, a Saturday I think, she came to bed having undressed in the bathroom as usual. I had come to know that these nights she'd usually do it to herself and was lying on my side, facing her bed, feigning sleep as normal and waiting for the fun to start.
True to form, after I had waited for several minutes I saw her bend and part her knees under the duvet. I saw the movement of her right hand go down to her. It wasn't the brightest of nights (unfortunately), but good enough to see nonetheless. However, unusually for her, I immediately noted that her movements seemed more urgent and faster than normal. She was certainly less cautious, with her glances towards me for example being minimal. Also, she didn't seem to be concerned that the duvet was moving and shaking as her hand worked away under it.
I sensed that she must have been really horny and aroused. That made me feel equally excited, so I was struggling to keep control and stay still. Also, her other hand was busy on her nipples and the movement had pushed the duvet down well below her breasts so I could see what she was doing. Her short nightie was fairly low cut and loose fitting so it was easy for her to get her hand to her breasts. She looked as if she was really squeezing and tugging at her right nipple with her left hand. She seemed to raise her head as if straining to get it as far off the pillow as possible and then I heard her gasping for breath.
I could vaguely see her mouth, which seemed to be wide open with her eyes screwed tight shut. As she came she slammed her knees together and the duvet started slipping to one side. As her orgasm finished I thought she'd settle down and go to sleep as usual, but oh no, I was in for a surprise. Instead, I saw her kick out with her left leg, presumably to try and re-straighten the duvet, but this didn't work. Instead she lowered both her legs flat onto the bed and then amazed me, as I saw that the duvet was still moving.
She was still playing with herself and her left hand was still busy on her nipple. I couldn't resist letting one hand slowly slip down to myself, as I realised she was going to try and cum again and without stopping from the first time. I could hear the duvet rustling slightly over her hand movements and I guess she realised that too, as she first glanced at me to check that I was still asleep, before kicking it to one side with her foot. Suddenly I could see her hand working at herself. Not clearly in real detail, but enough. Her short nightie had been lifted nearly to her waist and I could see her pubic hair, with her wrist resting on the lower part of her stomach, as her fingers bent and arched while working at herself.
Her movements weren't really rapid, more rhythmic, but it was obvious when she started to get close, as I saw her hips start to move and her pelvis seem to thrust upwards. Her breathing also become very audible, taking on a sort of gasping form. As she came I faintly heard her saying something, but couldn't make out what, as the noise of my heart pounding in my chest seemed to deafen everything. Shamefully that summer, my sister's sexual activities formed a major part of my motivation to masturbate!
Oh, and this is a great site - I'm enjoying reading other's experiences a great deal.