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

Step 2: The Quick How To

Use your nail or screwdriver to open up one end of the key ring and insert the end into the top hole of your zipper pull. Turn the ring until it is all the way on the pull.

Now you can use the ring as both a bigger, and therefore easier, pull tab AND a locking device. To lock your zipper securely 'up', pull the zipper all the way closed and slip the ring over the button. Put the button through the button hole as you normally would and....Voila

Step 3: Seamlessly Secure!

Voila! It's completely unnoticeable and completely secure! Now go forth into the world with confidence that your barn door will remain closed until you see fit to open it.