However, if you need to su - root after logging in, the X11 forwarding setup by ssh will no longer be valid.
ssh -X user@host
Do the following:
This will set the proper display. However, the Xauthority will not match. To fix that do the following:
<as user> $ echo $DISPLAY <note down the value> <as root> # export DISPLAY=<the value noted earlier>
# cp /home/<user>/.Xauthority /.Xauthority (or wherever the root home directory is)