I always give my clients DVD archives of the session files after a mix. Like others have mentioned, those files are practically useless unless the next user has all of the same plug-ins I used. I also second the notion that any "tricks" I used don't need to be kept a secret. In the hands of someone else, those tricks may not work.
I give them to the client because it is their responsibility to keep an archive, not mine. If they come back to me a year or two later and want revisions, etc, I don't want to be responsible for having the only archive.
Practice Your Mixing Skills!
Mix Our Tracks in Your DAW! www.Raw-Tracks.com