A momentary push-button ala Buckethead. As long as you keep the button pressed down, sound will be off. Let up on the button, sound comes back on. You can even go fancy and get buttons that light up when engaged, etc. Really any momentary push-button that has a solid feel without too much resistance will work. Just a button from Radioshack will do. But it has to really feel solid when you push down on it (at least when you're doing fast stutters with the kill).
I had one installed in one of my guitars years ago and LOVED it. I'm about to do it again to another guitar.