Create a file called ~/.ssh/save_ssh_agent.sh
on your remote box and make it executable:
#!/bin/sh
SSHVARS="SSH_CLIENT SSH_TTY SSH_AUTH_SOCK SSH_CONNECTION DISPLAY"
for var in ${SSHVARS} ; do
echo "export $var=\"$(eval echo '$'$var)\""
done 1>$HOME/.ssh/latest_ssh_agent.sh
ssh
into your box using ssh agent forwarding.