I put this together as part of my UnoJoy project, where we're using the Arduino as a platform from which to explore how we interact with video games. You can check out more of our projects for UnoJoy at unojoy.tumblr.com, or more of our other projects at the Exertion Games Lab.
Step 1: Materials
A pencil - the softer the lead, the better
Some 1+Megaohm resistors
Paperclips - make sure they're bare metal, not coated in plastic
Clear tape (optional)