NOTE: Depending on the time this can have up to a 35° error.

If you’re lost in the Northern Hemisphere:
Move your watch so that 12 o’clock is pointed to the left.
Then, move your arm so the hour hand points at the sun.
The spot halfway between the hour-hand and the 12 o’clock is SOUTH.

If you’re lost in the Southern Hemisphere:
Point your watch with 12 o’clock pointed at the sun.
The spot halfway between that and the current placement of the hour-hand is NORTH.