On OS X you can persist keys in ssh-agent by storing them in the built in "Keychain" application by using the -K option to ssh-add
ssh-add -K /path/to/keyBe aware there are some special caveats if you're using password protected SSH keys from older installations of OpenSSH (like on a previous version of OS X) when doing this on Mavericks. This Stack Exchange question covers the issue and has a solution.
Actually, make sure you
/usr/bin/ssh-add -K. E.g. you can have brew's version of OpenSSH and still useusr/bin/ssh-add -Kto add a key to the system's keychain