Quick tip: Making ssh agent work in screen sessions

The only annoying this I find in the otherwise indispensable GNU Screen is the fact that once you have launched screen (not resume) and have detached and logged off the first time, ssh-agent magic stops working in the screen sessions. Obviously this is because the next time you login, your ssh agent socket changes but the screen sessions still only have the location of the ssh-agent socket when you launched screen for the first time.