I followed the link that shuttleteam provided (BTW, thanks for that link), which took me to another link, which took me to another link, which took me to another link, which took me to another link, which took me to another link, which took me to another link, which took me to another link, which took me to another link, which took me to another link, which took me to another link, which took me to another link, which finally ended up wasting 2 hours of my life. Cool stuff. I've actually purchased $140 WLAN antennas :Q which may or may not work better than homebrew stuff. The Pringles Yagi antenna sounds cool, but a simple waveguide antenna may be better, plus it's easier and cheaper to make than the Pringles Yagi (imagine that!?!?!). One person benchmarked various homebrew antennas and found (in his particular application) that waveguide antennas gave better performance.
Tin can waveguide antenna
Dang it, now I'm wanting to try building my own.
wrap the notebook in foil
Bad idea.
get a card like an Orinoco that has a mini plug for an external antenna
Being able to use external antennas is probably the best way to improve performance. Few products support external antennas and different brands may have their own types of connectors, so choose the device before getting cabling/antennas. With the proper stuff you can shoot a signal for miles. I've seen claims of 20+ miles for 2.4GHz. My own experience has been just under 5 miles.