What to backup?

[Michael Kahle <michael.kahle@scc-wi.com>]

I just recently inherited a DLT1 tape drive from a friend!  :)  I ordered 4
DLT tapes and a cleaning tape.  I have downloaded and installed Arkeia to
use as my backup software.  My question for all is what should I backup?  Or
perhaps more importantly what shouldn't I bother backing up?  I have a ton
of data on this machine, but a small percentage of it changes frequently.

What I would like is advice on what things could I skip backing up?  I want
to do a rotation like the following:
Week1: Full backup first day, incremental the rest of the days.
Week2: Full backup first day, incremental the rest of the days.
[...]

I know that /proc is not necessary to back up, but what else?  I want to use
as little tape as possible (these tapes are expensive) and backup only what
is necessary to restore the machine to the state it was in before the crash.
I also want the data on the tape in case I delete something accidentally.

Thanks for your time.

Michael