Google search for wifi topics with these keywords:

yagi
directional
antenna
cantenna
pringles (although, pringles cans are not exactly the right size, but they do a good approximation)

You can also go read a lot about this at the Netstumbler forums.

You can also read up on the competition at Defcon this year, where the previous winners beat their own record with some god-forsakenly-big 6-foot satelite dish or something.

There is a ton of info on this...I've never done it myself, but I know it's out there and doable.