Freshman year of college, there was a coffee stand near some of my classes that I always went to... and for some reason I would always get raging erections while standing in line.
I'd pin it on the coffee, but it was only that one place.
And yes, the tuck is the way to go... just rotate to the 12 o clock position and tuck it under your belt. Just make sure nobody lifts up your shirt and sees your dick poking out.