Many people use something like denyhosts or fail2ban to help with brute force attacks. PF has built-in support for building rules with options which will throw potential attackers into a 'penalty box' based on certain factors like connection rate.
$ sudo iptables -A INPUT -i eth0 -p tcp --dport 22 -m state --state NEW -m recent --set --name SSH
$ sudo iptables -A INPUT -i eth0 -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 60 --hitcount 8 --rttl --name SSH -j DROP