Sometimes you find yourself having to redirect the X display to a different host but "ssh -X hostname" will not work since you had to switch users. For instance you logged to a host as root and afterwards "su - oracle".

Example:

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$ ssh root@host1.domain.com -X
# echo $DISPLAY
localhost:10.0
# xauth list
host1.domain.com/unix:11  MIT-MAGIC-COOKIE-1  95e4b887f2f6d132897aedbbbe297309
host1.domainom/unix:10  MIT-MAGIC-COOKIE-1  961e9e854127e3c70ff8804a5eb57f7e
# su - oracle
$ xauth add host1.domain.com/unix:10  MIT-MAGIC-COOKIE-1  961e9e854127e3c70ff8804a5eb57f7e
xauth:  creating new authority file /home/oracle/.Xauthority

Then trying xclock or xterm worked for me. If you still have a problem also try:

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$ export DISPLAY=localhost:10.0