there's nothing like a static location for SSH_AUTH_SOCK

Place in your .bash_profile, and your gnuscreen sessions will resume working when a new agent comes along (such as after log out).

#!/bin/bash 

# If you're on a multi-user system you might consider choosing
# a more secure (permissions-wise) directory than just $HOME/.
# Though it is just a symlink, and the destination of the symlink should already be secure.


# If it's not already our canonical file, and it exists... make it canonical:
[[ "${SSH_AUTH_SOCK}" != $HOME/.ssh-agent-$USER-screen ]] || \
    [[ -f ${SSH_AUTH_SOCK} ]] && \
         ln -sf ${SSH_AUTH_SOCK} $HOME/.ssh-agent-$USER-screen

# Other end of the link:
export SSH_AUTH_SOCK_ORIG=${SSH_AUTH_SOCK}
export SSH_AUTH_SOCK="$HOME/.ssh-agent-$USER-screen"