OS X (Apple)

Mac OS X 10.6: Leopard - I'm trying to create a script that will run on its own that can edit system preferences. I've done some digging, and found that a lot of system preference options are controlled by .plist files in ~/library/preferences, /library/preferences, and /system/library/preferences. However, I have yet to figure out how to edit certain functions. Specifically:
Turn on/off file sharing
Change screen saver
And some other functions like those.

My problem is twofold: For one, I can't find the file sharing plist file, and, secondly, I can't figure out a way to edit the plist files such that it makes any difference. For example, I found the plist file for screen saver, but try as I might, I couldn't get editing it to do what I wanted. And this was using plist editor. I'd imagine that once I try to port this to command line, it'll be even harder (presumably using the defaults write command). Any suggestions?

The main caveat with what you want to do, is once you set a preference the end user can just change and/or delete your property list file (.plist) and you will just be back to square one.

A lot of what you can do, you can do via MCX. Here is what I would do in a nutshell, and I am sure if you Google this you will find better examples.

1) - download server tools
2) - Configure you client with Work Group Manager on the local Directory Services node
3) - Manage the computer exactly how you want to with the server tools running locally
4) - Once it is set up how you like it, export the MCX file for that user via dscl (directory services command line), and then you can write a log in hook script that imports that file when a user logs in

MCX cannot be modified or delete by the end user. Since Directory Services is very similar local client side as it is server side you can do this. The other option is to set up OS X Server running OD and then bind the client to the server and manage it that way.