Intentionally helping someone poke a hole through their firewall to allow any users on a remote machine access to their computer is generally a bad idea... unless they want you to help them with their computer.
In that case, I find it really useful.
Toward this end, I've setup a dedicated user account (email@example.com in this example). I granted myself shell access to this user and generated an ssh key pair:
ssh-keygen -t rsa
Then, as a convenience I move my public key into the home directory:
cp ~/.ssh/id_rsa.pub ~/
Next, I give the user I want to help ssh access to this shell account on an Internet connected computer (in this example: firstname.lastname@example.org). Either I share the password with them or if they have an ssh key or monkeysphere identity I use that.
Then, I ask them to grant teh jamie-share user access to their user account by running (which downloads the key I just created):
mkdir -p ~/.ssh ssh email@example.com "cat id_rsa.pub" >> ~/.ssh/authorized_keys
Next, ask them to install openssh-server on their local computer:
sudo apt-get install openssh-server screen
Next, they run the following command on their local computer:
ssh -R 2222:localhost:22 firstname.lastname@example.org
This command says: forward port 2222 on chavez.mayfirst.org to port 22 on your local computer.
Lastly, I log in to email@example.com and run, replacing with their local username.
ssh <their-username>localhost -p 2222
You can do whatever you want now, but running screen is a good way to share a session so the person you are working with can see what you are doing as you do it.
p.s. Thanks Ross for the tips!