backup with tar: Which dirs

[v.demartino2 at virgilio.it]

I have a large usb harddisk with a 60GB freebsd slice that I want to use
to backup the FreeBSD 5.4 installation on my laptop once in a while.
I think that tar is the right tool for this purpuse.

Now I ask:
What directories do *** NOT *** require to be backed up?
I know this is the case of /tmp 
but what about /dev? and /var?

Ciao
Vittorio