[newbie] X-Server Problems

[Paul]

Hi,
Whenever I su in as another users, then try to run an x application, I 
get the following message:

Xlib: connection to ":0.0" refused by server
Xlib: Client is not autorised to connect to Server
Krite: cannont connect to X server :0.0

How can I solve this?  It's a pain having to log out, then in as root 
just to edit some text files, or update certain programs :(

Any help appreciated.