This is one of the easiest and cheapest solutions I've ever heard for a problem that can potentially cost you a lot of embarrassment and social awkwardness. For 10 cents you can ensure that your fly is never down in public again!
Step 1: Supplies
- one 3/4" OD double key ring (just larger than the button on your pants)
- really strong nails or a small screwdriver
- pair of pants with a constantly low flying zipper