Help with permissions to allow adding files to a directory but not modify any

From: Todd (jensen5_at_optonline.net)
Date: 06/28/04 

Date: 28 Jun 2004 13:44:05 -0700

I can't find the right combination of permissions to let someone add
files to a directory and at the same time preventing any modification
to the files once placed inside. Create Files/Write Data permissions
seem to be better if it could be split up. Can anyone give me the
right permission combination to do this.

Thanks