Getting network-manager to forget
I made the mistake of associating with the access point in the hotel across the street from the office once to do some testing. From then on, network-manager seemed to prefer that AP, perhaps because the office APs have security enabled. There doesn’t seem to be a UI to make it forget that it likes a certain network. To do so, run gconf-editor and navigate to /system/networking/wireless/networks. SSIDs are folders within this; navigate into the one you want to forget and unset all the keys inside it.