/etc/interfaces:
...
iface wlan0
inet dhcp;
 wpa-conf /Pfad/zur/wpa_supplicant.conf;
auto wlan0

/Pfad/zur/wpa_supplicant.conf:
network={
        ssid="Netzwerkname"
        scan_ssid=1
        proto=RSN
        key_mgmt=WPA-PSK
        pairwise=CCMP
        group=CCMP
        psk="meinschluessel"
}