Here's what I have setup in the past, which does allow me to access my Mac via ssh:
On my AT&T gateway, I setup the firewall settings to push all internet traffic directly to my Airport Extreme. While the Airport Extreme is by no means an enterprise class firewall, it actually works really well for home/small office security needs. I then use the Airport Extreme to send all traffic on port 22 to the internal IP address of my Mac mini. I have my DHCP settings on the Airport Extreme setup with a static map, which always gives the same IP address to my Mac mini. I then use Terminal, and type in the following:
After this, I'm asked for the password of my user name. Once I put in the password, Terminal then gives me a prompt for the user account that I logged in as through ssh.
Keep in mind that by sending all ssh traffic directly to your home system you are opening up a big security hole. Even with a strong password, you're still at serious risk. I've only done this on a few occasions, and for very specific reasons. When I used to run OS X Server at home, I had ssh forwarded directly to my server. The server's logs showed that I was constantly under attack. People trolling the internet had found my server, and they were constantly trying to break in. After I noticed that, I setup VPN so as to have one more barrier between my server and the outside.