how to use x11 forwarding?

Sorry for the newbie question, but how do I get X11 forwarding to work?

I want to use ssh to connect to a server (running openssh 3.9p1 on 
Solaris/x86), and run a program, having it's display appear on my client 
(PowerBook laptop running OSX 10.3.5 running openssh 3.6.1p1).

On the server I set the X11Forwarding parameter to yes in sshd_config 
(and restarted sshd) and on the client I set ForwardX11 to yes in 
ssh_config.

If I try and run an X based program it complains because the display is 
not set (Error: Can't open display: ).

Normally, the client is running on a DHCP network on an ADSL broadband 
connection with a really crappy restrictive firewall that I can't do 
much with. The server is behind a firewall that I have full control over.

Right now I am trying to do this with the client and server behind the 
same firewall to take that out of the equation but I can't get it working.

How do I need to configure the ssh client and server to achieve all this?

As a test I telnetted to the server, set the display to the mac's ip 
address, ran Apple's X11, set the xhost on the client to allow 
connections to the server, then ran admintool and it appeared properly, 
so I know the X server connection is working properly. However I want to 
get the SSH tunneling working so I can do this from home too.

Thanks

Chris