A nice script to do that is mkstage4.sh.  I've been using it to backup
my system for a while now, and it seems to work fine.  <grin> Granted I
haven't borked things up badly enough to require a re-installation in a
while.  I'll attach it here, so you can take a look and modify it for
your uses.

Hope it helps.
On Tue, 2007-04-24 at 12:00 +0100, Peter Humphrey wrote:
> I'm considering experimenting with other file systems, JFS in particular. I 
> want to use a spare disk area and install an exact duplicate of my present 
> system, which lives in ext3.
> 
> The straightforward way seems to be to copy the world file from the current 
> system to the new one (after finishing a basic installation using the 
> standard methods) and running "emerge -uaDvN world". Is this likely to run 
> me into problems? Perhaps I should prefer "emerge -1 `cat old.world.file`?"
> 
> Another way would be just to tar up the old partition and untar it onto the 
> new one, but I think I'd feel happier installing into place. Seems cleaner, 
> somehow.
> 
> -- 
> Rgds
> Peter Humphrey
> Linux Counter 5290, Aug 93