There was a previous tip that showed how to remove a network from your list of preferred network so your Mac doesn't automatically log in to this network in the future. The main reason why I used this was because way too often, my Mac wouldn't connect to the network I wanted. There is a better way of handling this for situations where your Mac finds many of your preferred networks. You can basically drag the networks around on your list so that the networks that you use the most are at the top of the list. That is the way your Mac chooses on which network to log in. It depends in which order they are. You can access your preferred networks list in the Advanced Airport preferences.