How do I clean up /var?

["Andrew" <knoppix@rngresearch.com>]

Dear Klaus et al,

I'm running off usb stick with persistent image and the system seems to
have slowed down considerably since install.  It is getting annoyingly
slow.

I tried deleting many of the audio files, especially the larger ones, from
my home directory, and recompressed many others with higher compression. 
I noticed an immediate speed improvement.

Now, I discovered over 800MB in /KNOPPIX-DATA/var.

About 99.9% of the data resides in the tmp, lib, and cache subdirectories,
with about one third of the data in each.

What can I safely delete from there?

Thanks!

Regards,
Andrew