When a little boy gets his penis stuck in a zipper, it makes me cringe.
The August 2001 issue of Pediatric News contains a great tip from San Diego emergency room physician Dr. John Kanegaye: a plentiful coat of mineral oil on the injury eases the disengagement of skin from zipper.
I suspect it is also a little soothing in the meantime.
Dr. Kanegaye also recommends depilatory wax for removing cactus spines from the skin (hopefully other skin than the penis). Coat the entire area with wax, and then pull the whole thing off.
I love simple, practical solutions for minor emergencies that we see in the pediatric office.