First open up a terminal and type -
ifconfig
Then you'll get output that looks something like this:
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=4219b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,TSO4,WOL_MAGIC,VLAN_HWTSO>
ether bc:5f:f4:fb:a2:a9
inet 192.168.3.6 netmask 0xffffff00 broadcast 192.168.2.255
nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
media: Ethernet autoselect (1000baseT <full-duplex>)
status: active
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
options=600003<RXCSUM,TXCSUM,RXCSUM_IPV6,TXCSUM_IPV6>
inet6 ::1 prefixlen 128
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x2
inet 127.0.0.1 netmask 0xff000000
nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
The important things to look for here are :
em0 (your network device mat be different - adjust accordingly)
inet 192.168.3.6 and
netmask 0xffffff00 (on your system your numbers may be different so adjust accordingly)
Then log in as root
Then type -
ifconfig em0 inet 192.168.3.6 netmask 0xffffff00
(from your terminal output)
then still as root:
dhclient em0
(again - adjust accordingly)
The output from this command will contain something similar to:
DHCPACK from 192.168.3.4 - (your number may be different) remember 192.168.3.4 for when editing /etc/rc,conf
then the command:
route add default 192.168.3.4
-(or your number)
then edit /etc/rc.conf:
command:
ee /etc/rc.conf
Go down with your keyboard arrow keys to the line:
ifconfig_em0="DHCP"
and comment (#) this line so it looks like this:
#ifconfig_em0="DHCP"
then under this line enter these lines:
ifconfig_em0="inet 192.168.3.6 netmask 0xffffff00"
defaultrouter="192.168.3.4"
Make sure you use exactly the same numbers as your terminal output above and be careful to put correct spaces and quotation marks.
then press Esc save changes and reboot and you should have your static ip address working and slightly faster boot times.

If for some reason you want to go back and undo all these changes then:
Open a terminal, log in as root:
Then type :
ee /etc/rc.conf
Go down with your arrow keys to the 2 lines that you made before:
ifconfig_em0="inet 192.168.3.6 netmask 0xffffff00"
defaultrouter="192.168.3.4
and comment them (#) so they look like this:
#ifconfig_em0="inet 192.168.3.6 netmask 0xffffff00"
#defaultrouter="192.168.3.4"
Or you can just delete them
Then go to the line :
#ifconfig_em0="DHCP"
and uncomment it so it looks like this:
ifconfig_em0="DHCP"
Press Esc and save changes when prompted and reboot and it will back to the way it was before