Getting X11 forwarding through ssh working after running su
$ xauth list $DISPLAY
You'll get something like
somehost.somedomain:10 mit-magic-cookie-1 4d22408a71a55b41ccd1657d377923ae
Then, after having done su, tell the new user what the cookie is:
$ xauth add somehost.somedomain:10 MIT-MAGIC-COOKIE-1 4d22408a71a55b41ccd1657d377923ae
