This happened a few years ago over Christmas break.
I was 15 at the time and one of my best friends and I were in Martial Arts and always enjoyed going out on my trampoline and sparing and wrestling to practice for competitions. He had a younger sister (not going to use real names so we'll just call her Sarah) that would occasionally tag along with us (they lived across the street) because she had a crush on me. This particular winter afternoon she was with us. She was a couple years younger than me, petite, bright blonde wavy hair, blue eyes, cute face, and a tight muscular build for her age (she was in gymnastics for several years).
So... continuing with the story. My friend and I were practising some JuJitsu on the trampoline for about an hour when his mother called and wanted him home to take care of some things. He left with the thought that he would be returning in a few minutes. So I was there with his sister alone on my trampoline. I was really exhausted at that time (we had been wrestling for over an hour) so I just laid down on the trampoline to rest while I waited for my friend to return. The next thing I know Sarah is lying on top of me just relaxing. She only weighed about 70 lbs so I didn't think anything of it. After about 10 minutes she gets up and starts walking around the trampoline. Then I notice my legs being moved around and dropped in a slightly spread out position (again I was too tired to think anything of it). She then proceeds to place her feet (sock footed) in between my legs. She closes my legs on hers so that my feet are at her hips, and begins to use her toes to massage me 'down there' at this point I was half way asleep and didn't fully realize what was going on.
After a while of it I became aware enough to realize that her feet were creating a VERY pleasurable feeling throughout my entire body. By then, however, I was enjoying it so much that I only wanted it to continue, so I pretended to still be asleep. She continued this for about five minutes, slowly pressing the pads of her feet and her toes against my enlarging cock in a soft, smooth motion. Then she proceeded to sit up and straddle me and lay down on top of me so that the tent in my jeans was pressed right against her pussy through her velour pants. She laid there for a short while then sat back up, nestled herself back between my legs and placed her hands on me.
She began to play with my cock through the jeans by walking her fingers from one the base to the tip squeezing lightly with two fingers as she progressed. She then started to unbuckle my belt with one hand while still caressing me with the other. Following that she slowly unzipped and unbuttoned my pants and then pulled them open so that my boxers were the only thing between her hands and my cock. She proceeded to open the slit in the front of my boxers and put her fingers inside to grab my cock. She then grabs the waist line of my boxers, lifts it, and pulls them down just enough to get a glimpse of what lied beneath (haha). Then she lays back down on top of me, this time facing up, and (assuming I am still asleep) picks up my hand, and places it right on top of her pants over her pussy. So I am just laying there with my jeans opened and my hard cock pressed against her tight butt (through clothing) with my hand on her pussy. This was not the way I planned to spend my afternoon (haha). But it gets better still. She then lifts my hand and lifts the waist line of her pants and places my two fingers right against her panties and starts using them to slowly rub her pussy. She had no hair down there and her panties were silky and thin so I could feel every detail of her soft, smooth pussy as she took deep breaths and massaged herself with my fingers. Unfortunately after about five minutes her brother calls her from outside my backyard for her to come home which puts an end to the fun and therefore my story.
Needless to say, this is the memory that causes me to get off whenever I need to.