before you become su,get your MIT-MAGIC-Cookie

xauth list $DISPLAY

you’ll see an output like
myhost.mydomain:21 mit-magic-cookie-1 4d22408a71a55b41ccd1657d377923ae

switch to root (’su’)

add your cookie to the session
xauth add myhost.mydomain:21 mit-magic-cookie-1 4d22408a71a55b41ccd1657d377923ae

that’s it, now you can start your forwarded X11-Application

sometimes you have to set your $DISPLAY variable,so copy the value from ‘echo $DISPLAY’ before su and set export $DISPLAY=”myhost:XY” when you’re root