Via XDMCP
XDMCP is the X Display Manager Control Protocol. In order to use it, you will have to log in via a graphical login manager (GDM, KDM) on the machine which will act as the server. Configuration steps require 'root' privileges and have to be done on the machine running the login manager only.
 
Edit '/etc/X11/xdm/xdm-config' and put an exclamation mark in front of this line:
 
 DisplayManager.requestPort:     0
Edit '/etc/X11/xdm/Xaccess' and remove the hash (#) in front of this line (if there is one):
 
*     # any host can get a login window
Edit '/usr/share/config/kdm/kdmrc' and change
 
[Xdmcp]
Enable=false
 
to
 
[Xdmcp]
Enable=true
 
If you're using GDM, you can configure that in the advanced options field of gdm-config.
 
Log out of your X session and log in again.  
Now go to the client machine, log into a console and run
 
X -broadcast
 
(notice that you can and should do this as non-root). The login screen of the remote machine running the display manager will come up and you can log in.
All applications will run on the remote machine and the visuals will appear on the local machine via the network. To quit the session, log out and choose 'Close Connection' from the 'Menu' dropdown.
 
Since the local X server requires very little resources, this is a neat way to run a full blown graphical desktop on older machines.
 
Security notice: Only use XDMCP in trusted environments, there's no encryption whatsoever, everything - including passwords - is sent in plain text over the network between the connected machines. XDMCP requires UDP port 177 to be open on the display server.