From the department of Random and slightly useful things to know: I was hiking out in the middle of nowhere one day when the button on my favorite camping pants just plain fell off. Fortunately for me, I had a key with me.
Step 1: Just Do It.
Once again, The pictures explain a lot. Make a small hole where the button used to be. Take your key and thread the key ring through the hole. The key fits through the button hole sticks out. You can fold it under a belt loop or run the key from the outside-in and just tuck it down. You will be the coolest person ever .