Edit: If you have all the ssh keys and hosts properly set up, you can just do
ssh -t intermediary_host ssh -t target_host
If you need SSH access to a machine that's either behind a NAT or in a network ran by a paranoid sysadmin, you'll need a third machine with public SSH access (hereby referred to as intermediary).
At the target machine (the one you can't access directly via SSH) run:
$ ssh -v -R 12345:127.0.0.1:22 user@intermediary_host
Then SSH to the intermediary and run:
ssh -v firstname.lastname@example.org -p 12345
Et voilà! You can ditch the
-v flag to avoid verbose output.