Adding a new swap file in FreeBSD:

	- Create the file:
		% dd if=/dev/zero of=/usr/swap0 bs=1024k count=512
			(Would create a 512MB file at /usr/swap0)

	- Use it without rebooting:
		% mdconfig -a -t vnode -f /usr/swap0 -u 0 && swapon /dev/md0
			(Mounts the file as the /dev/md0 device)

		% swapon /dev/md0

	- To enable on boot, edit /etc/rc.conf:
		swapfile="/usr/swap0"

Use swapinfo to see your swap devices.