Sad but true... Keep everything up to date, subscribe to your distro's security list. Back up /home and don't run as root. Use sudo instead of su where you can. And above all set up your firewall properly if you''re not behind an external firewall.
Be smart with spam, and it should all be good. A well configured and maintained Linux box is a pretty tough target if you're just a tad paraniod.
Just my $0.02