I learned how to do this in a cooking class long ago. I don't remember any more exactly how I used to cut up a bell pepper, but I know it wasn't this easy and uniform, so I figured I'd share. It helps to start with a squarish, or at least not bendy, pepper.
1. First, lop off the top. Don't worry if half the stem is left behind in the pepper.
2. Turn the pepper so it's sitting on that nice stable flat side you just made. Then you can cut straight down to slice off a side. (This is where the "squarish" part makes it easier.)
3. Continue cutting off the sides straight down. You'll end up with something sort of like this.
4. Turn what's left of the pepper over on to its side and cut off the top. If your sides of the "pepper cage" around the seeds are thin, it might break in half like mine did.
5. Then it's easy to slice and/or chop up the conveniently sized and shaped pieces of pepper for cooking or just plain eating. Yay!