edit themes for client review

I am using wordpress for my clients websites. How do I keep the current theme active and show my client a new updated design using another custom theme or another version of the same theme?

I do not know how to do this without installing another mySQl wordpress.
The problem I am having is keeping the current info linked to the new theme.

Is there a plug in I can use? Or is there a workflow to follow on how to do this?

I am currently installing wordpress on another site and I am calling it mysite.com/test/myclient. I make changes to the theme and place dummy text & images. Then I copy the theme and place it into my clients wordpress theme folder. Then activate it and I am done.

This work flow is ok but a little time consuming. There current info is not being used b/c it is on a separate database.

I would love to be able to edit an inactive theme and preview it in the browser. i use coda and xyle scope to edit my code.

Thanks.