I think Dalebert really hit the nail on the head there :p
Alot of it is about you just feeling confident, or atleast, not worrying and working yourself up about things.
Though some stps are not cheap, I think the one I have, which Ive had for 4 years now, cos about £60, (which I saved for ages for :p), Im glad I got it and if people are serious about it/peeing in public is a big deal to them, investing in an stp, especially a good, passable one, is a good idea!
The one I have is the Mango one, (though warnings to anyone using their site, their service is NOT good) I use it as a "packer" too, and it passes enough in public bathrooms. I mostly use it on nights out as I know I will be needing to go to the toilet alot once Ive broken the seal! :p
Just Tuesday night I was out, and in the club toilets, instead of having singular urinals, they had the one long open one that spans two whole walls! I used that, (I go for the corner/end, if no corners are free, Ill use a stall. If no stalls are free, Ill wait for one and moan, drunkly about it being too busy :p).
However, it took ages to build up my confidence to this.
Ive never had any problems with using urinals, or using stalls for that matter.
I did have one guy once, seem to look over, and I think he may have noticed something peculiar about my "downstairs", but instead of panicking, thinking, crap, he knows! I just glared at him for looking at me, and the tables were turned :p