Hold a watch with 12 o'clock at left. Move your arm so the hour hand points at the sun. The spot halfway between the hour hand and the 12 is south.The comments of the Wired post offer a few important caveats. First, if you're in the southern hemisphere, you would point 12 o'clock at the sun but still split the difference between 12 and the hour hand for north. Also, if you practice daylight saving time, you should subtract one hour from the hour hand.
Use a Wristwatch As a Compass [Wired How To's via MAKE]