automatically moving emails from the Sent folder to another

[Jane Jordan (gmail)]

Hi there.
I've got a folder of emails to and from a person.  I want to make  
sure that emails from the Sent folder are moved to that folder.   
Emails that are downloaded automatically get moved, but how do I see  
to it that emails in my Sent Mail folder to her get moved  
automatically as well.  For now, I have to go to the Sent Mail  
folder, go to Move Message, and then select the appropriate folder.   
Can this be fixed with a rule, and if so, how do I do that?

Thanks a lot.

Jane