Turn on x11Forwarding in your sshd_config file - there's a section in
here that talks about X Forwarding.
Edit: or wait, did you mean you'd like to control the whole desktop, not just a single application? If you're looking for the whole desktop and don't mind VNC, might I recommend
x11vnc? It's one of the (only?) very few VNC servers that lets you access :0, the default, already-in-use desktop, rather than creating a new one for each incoming connection.