[kijin]

A wok (Chinese stir-fry pan) works surprisingly well for this purpose. It's roughly parabolic, and has a smooth surface that helps you avoid irregularities in the shape of your foil.

It gets even better if you drill a hole in the center and stick an antenna vertically in there. Now you've got a wi-fi antenna with all the advantages of a satellite dish! The Chinese call it wok-fi.

[westmeal]

Huh... no kidding. I'm going to have to try that out some day. How was the signal like on the wok with the antenna through the center?

[kijin]

I haven't personally tried sticking an antenna through a wok (just putting one behind the router was enough for my purpose), but rumors say that you can achieve a range of several miles if you do it right.

[walshemj]

There are online calculators http://www.radiolabs.com/stations/wifi_calc.html note that the cable loss mentioned is the antenna feeder cable

And Cisco Press do some good books on the basics of WiFi network design.