Edit /etc/rc.conf and add something like:
cloned_interfaces="vlan23 inet dmz steak" ifconfig_em0="up" ifconfig_bce0="up" ifconfig_vlan23="inet 10.23.1.1 netmask 255.255.255.240 vlan 23 vlandev em0" ifconfig_inet="inet 224.225.226.227 netmask 255.255.255.255 vlan 1 vlandev em0" ifconfig_dmz="inet 10.42.2.3 netmask 255.255.255.0 vlan 42 vlandev bce0" ifconfig_steak="inet 10.10.10.254 netmask 255.255.254.0 vlan 69 vlandev bce0"