I get this really bad sometimes, if I'm stressed, I just won't be able to go if there is someone next to me at a urinal. Or if there is a line, my bladder tenses up too (like at a game), I guess from the 'performance anxiety". If I even think there is a chance this might happen, I use the stall. Nothing worse than standing around with your dick in your hand next to another dude, while there is awkward silence from the fact that I'm just standing there, not pissing.
If I'm not stressed, I'll practice just letting my mind wander while I piss at the urinal with others around, and that usually gets me flowing. Over the years, my pee shyness has gone down with practice, but if I'm stressed, or if it's one of those trough deals, forget about it! I'm using the stall. I wish I could just whip it out and let it all hang out while I relieve myself, like the other guys, but my shy bladder won't have it!
That is, of course, unless I'm drinking beer, then I can whip it out and piss on anything in front of anyone! That's usually when I'll take my time letting it drain, since I have to piss like forever then, and I won't care if people come and go, or if someone's blatantly checking out my dick, that still won't make me pee shy. I always feel like beer is a good temporary cure for stage fright at the urinals.
But I rarely ever drink, so the rest of the time, I have to deal with a shy bladder, and it sucks!