riddlermarc said:Get some foil and make a shield in front of the aerial, this will stop excess signal pointlessly leaving the front of your house and help focus any signal coming in from the bottom of your garden (assuming back garden)..
There was an excellent thread in here last year about making your own parabolic antenna with a wok or something, now the search facility is gone I couldn't find it.
EDIT: http://www.usbwifi.orcon.net.nz/
Also check out http://www.techtastic.ca/articles/homemade-antenna.html