[Orgmode] How to backup the DONE items?

[Feng Zhou]

Hello, all
   I am using org mode, but as time goes on, my org file becomes
larger and larger. So, I want to move all the DONE and Archived items
to another file (preferably with a copy of the headers that contain
them for reference), but I don't know how to do this.  Any help on
this? Thank you.

--Feng