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Don't expose it directly to the Internet, such as by assigning it a publicly routable IP address or forwarding the ports it uses (typically 515 and 9000-9003/TCP) from hosts on the Internet.

For the most part, if it's on your "internal" network and isn't accessible from hosts on the Internet, you're safe (except from "insiders").

If you need it exposed to the internet you can at least attempt to change your port forwarding settings on your router to use a different port on the public side of the connection, for example, routing port 10000 to 192.168.1.x:500 or whatever the case is. Not sure why you'd want to print to your home/office printer while you're not there though.

Firewall.

Exactly. http://en.wikipedia.org/wiki/Firewall_(construction)