/boot/loader.conf

# This is loader.conf - a file full of useful variables that you can
# set to change the default load behavior of your system. You should
# not edit this file! Put any overrides into one of the
# loader_conf_files instead and you will be able to update these
# defaults later without spamming your local configuration information.
#
# All arguments must be in double quotes.
#
# $FreeBSD: src/sys/boot/forth/loader.conf,v 1.85.2.2 2005/03/07 17:03:18 des Exp $

# Splash screen
splash_pcx_load="YES"
vesa_load="YES"
bitmap_load="YES"
bitmap_name="/boot/splash.pcx"

# Sound drivers
#snd_driver_load="YES"

# Load wireless drivers
#if_ath_load="YES"

# FS drivers
udf_load="YES"
ntfs_load="YES"
ext2fs_load="YES"

# cpufreq(4) framework, required for powerd(8)
cpufreq_load="YES"

# Tunables
kern.maxfiles=24576

network default

If your wireless network has a dhcp-server and you get your parameters dynamicly when connecting, just add this line into your /etc/rc.conf file:
ifconfig_ath0="DHCP"

And if you specify parameters by yourself, add this lines:
defaultrouter=""
ifconfig_ath0="ssid inet netmask "

and to your /etc/resolv.conf:
nameserver

And that's it, you would not need to do anything manually anymore.