Once you have a working RSA key pair, the next step is to write a little script to get ssh-agent running. I found one called check_ssh_agent.sh that I got from here. (This site also has some details about how to do a setup similar to the one I'm describing here, but he does things a little differently.) I put the script in my home directory here: ~/Documents/scripts/check_ssh_agent.sh.
Make sure the file is executable:
chmod 700 ~/Documents/scripts/check_ssh_agent.sh
Finally, you need to add an environment variable called SSH_AUTH_SOCK which points to the socket that ssh-agent uses. You also need to add the scripts folder to your PATH and add a line to launch the check_ssh_agent.sh script when you login. To do this, edit your bash profile:
Add the following:
(note: if your path has other stuff, leave it and just insert the scripts directory, separating it from other directories using :'s)
Relaunch terminal. Then, before you do your next ssh authentication, do:
This will add your private key and you won't have to type your passhrase again until you reboot.