Other OS info

I haven't used it yet but it looks like a good way to keep the bad guys out. Of course, changing the default port always helps quite a bit too for the automated scripts out there.

SSHd Filter

Fail2Ban - automated prevention help

Blocks IPs after it has determined that the IP trying to get in is a 'bad guy' - great for SSH servers and other services that are commonly attacked brute-force style.

A great HOWTO writeup is here:
http://www.the-art-of-web.com/system/fail2ban/

You could always get real clever and do port knocking but that is sometimes a hassle.