The way I know is using an instance.

Create an object with origin in (0,0,0), divide object in two sides, delete one side.
Create an instance (Alt + Dkey).
Esc key to don't move the instance, Mirror the instance (set cursor to 3D).
Modify the original object.
Last join two objects and remove duplicate vertex.

Notice you can also mirror vertex instead of creating another object.

Message edited on: 09/12/2005 16:37