/etc/conf.d/net:
modules=( "wpa_supplicant" )
wpa_supplicant_ath0="-Dwext"
wpa_timeout_ath0=20

/etc/wpa_supplicant/wpa_supplicant.conf:
network={
   ssid="my-ssid"
   psk="my-psk"
   priority=5
 }