This is how I did it once!
I was on a bus, yes a bus, and this really fit girl was staring at me. She was wearing a low cut top, so low that I could almost see her nipples. She didn't seem to mind me staring and I soon became really hard. I think she could see this because she laughed to herself, and she winked at me when she got off. There was just me left on the top deck of the bus. It stopped in its usual place so the driver could get out and have a cigarette.
I was sitting with my hand down my trousers gently rubbing my cock while thinking about her, and the greatest urge to wank came across me. I had some tissues in my bag, which was good. I began to gently push and pull my foreskin over my head underneath my jeans. My boxers were rubbing against my dick, which felt great. After a while I could feel myself getting close until I finally came inside my pants. It was nothing like I had ever felt before. The fear of being caught really intensified the orgasm. The only problem is I never get on that bus anymore, just in case someone saw me!