Start ssh-agent when necessary and add keys on first use. Add the following to your .bash_profile.
# Ensure that we have an ssh config with AddKeysToAgent set to true
if [ ! -f ~/.ssh/config ] || ! cat ~/.ssh/config | grep AddKeysToAgent | grep yes > /dev/null; then
echo "AddKeysToAgent yes" >> ~/.ssh/config
fi
# Ensure a ssh-agent is running so you only have to enter keys once
if [ ! -S ~/.ssh/ssh_auth_sock ]; then
eval `ssh-agent`
ln -sf "$SSH_AUTH_SOCK" ~/.ssh/ssh_auth_sock
fi
export SSH_AUTH_SOCK=~/.ssh/ssh_auth_sock