I was using a combination of a VPN and NAT rules to do so.
Basically, machine A (the machine behind the firewall I want to connect to) would connect to machine B (a VPS or AWS instance - free tier micro instances are awesome for that matter!) using a VPN connection (pptp or openvpn or l2tp or whatever - pptp has the advantage of being super easy to setup and working out of the box on most linux distros. Not the most secured though, but to run SSH on it, it's good enough). I had a script to periodically check (every 1 minute or so) if the VPN connection was up and if not, try to reconnect.
Then, I had some iptables NAT forwarding rules on machine B (let's call it gateway) to send all TCP traffic on a defined port to the machine A, port 22, using the VPN interface.
It had the advantage of being quite easy to setup for me as I'm quite used to setup VPNs and NAT forwarding rules (for having living in China, bypassing firewalls is almost an everyday routine exercise :) Also, it worked perfectly well and the performances were reasonable. I could access my server at home, in Beijing, behind a NAT, a dynamic IP and the country's firewall, from anywhere in the world. I was happy!
There are surely other (better?) ways to do it though, and the autossh/reverse tunnels option looks very interesting.