How to enable faster WiFi Roaming with Mac OS X & Airport base stations
You can change the system preferences for JoinMode and JoinModeFallback to be the following:

    JoinMode (String)
        Automatic
        Preferred
        Ranked
        Recent
        Strongest
    JoinModeFallback (String)
        Prompt
        JoinOpen
        KeepLooking
        DoNothing


Do this using the airport command: 

/System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport


Run the command to see the options, up the top you'll see a section on how to sec preferences.

For instance:

sudo /System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport prefs joinMode=Strongest


Note, this preference isn’t permanent, so you may need to automate this or issue it for specific problem situations or network providers.