Go into System Prefs/Network and select Airport in the Network Status window. The dropdown that says "By default, join" - select "Preferred Networks"
You can add them in order of preference to the panel below this dropdown so whenever you are near one of the preferred, it will automagically attempt to join.
-= You haven't lived until you have ridden a 90kph hairpin with your knee scraping and nothing but a few millimetres of cowhide between you and the asphalt. =-
iPad 32Gb WiFi
MacBook 2.0GHz C2D/2Gb/160Gb
MacBook Pro 2.2GHz C2D/4Gb/320Gb
iMac 20" 2.0Ghz C2D/4Gb/250Gb + 23inch Cinema Display (DVI)
16Gb White iPhone 3G