Before me and my Ex GF had sex for the first time (not mine or her FIRST time, just first time together), I went round her house to watch a movie, we cuddled and kissed while watching a film and I had the MAJOR horn, I asked if we could have sex but she said no and sent me home. Her dad was In the house too and he didn't want me to stay.
When I left, I was walking up through her back garden and I noticed I could see right into her room, more importantly, right onto her bed (she had one of those high beds with a desk underneath) I watched her for a while, thinking how lucky I was to be with her then I noticed some pretty distinctive movements near the crotchal area, sure enough, she was strumming away as soon as I left!!! (probably making me wait, but was equally horny ;-))) I wasn't going to pass up this opportunity so I got mine out and started wanking away, I came in what seemed like 5 seconds and left a mess on the patio.
I don't speak to her anymore but she never did find out ;-)
Also have done it at my desk at work, in my room when my mates were round (I was in bed) while walking home (didn't even stop, just got it out) sick bay at school, best friends parents bed, swimming pool, back of a minibus on the way back from Newcastle on a night out, at a bus station, waiting for the bus... the list goes on