> auto wlan0
> iface wlan0 inet dhcp
>      wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf

     allow-hotplug wlan0
     iface wlan0 inet manual
          wpa_roam /etc/wpa_supplicant/wpa_supplicant.conf

# mapping for the default id_str of wpa_supplicant:
     iface default inet dhcp

     Plus some sections like the following if you have static addresses:

# mapping for id_str="foo"
     iface foo inet static
        address      192.168.0.14
        netmask      255.255.254.0
        network      192.168.0.0
        broadcast    192.168.0.255
        gateway      192.168.0.250