Additionally to the anomie post it is *invaluable* to install
denyhosts scripts, especial if it necessary for you to open the ssh service to the public internet.
It is available from the ports: /usr/ports/security/denyhosts
It blocks ssh user dictionary probes or other evil actions using tcpwrappers
I personally have configured the denyhosts to use my pf firewall as well ,that populates a related table with the attacker's IPs.
I am very satisfied with denyhosts, it blocks daily dozen of attackers.