These are available commercially, but I have yet to see a farmer who didn't think he could build it better himself

My winter job has one of the commercial versions setting in a window pointing toward the boss's house I guess 4-500 feet away. It seems to work OK, but he tends to gripe if we park a truck between it and his house at night.   

Go to http://www.turnpoint.net/wireless/cantennahowto.html

or http://verma.sfsu.edu/users/wireless/pringles.php

These guys offer commercial antennas that are supposed to work better, but not near as much fun as making something out of nothing http://www.hdcom.com/pringle.html

You can also do a Google search for Pringles can antenna for more info