/etc/network/interfaces
-------------------------
auto lo

iface lo inet loopback
iface eth0 inet dhcp

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

And the /etc/wpa_supplicant/wpa_supplicant.conf file
-------------------------
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1

network={
   ssid="gentoonet"
   psk="my_passphrase"
   proto=WPA2
   key_mgmt=WPA-PSK
   pairwise=CCMP
   group=CCMP
   priority=5
}
-----------------------------------