How to properly turn off REVISIONS and AUTOSAVE for whole site and optionally for a custom post type only

Question

Is there a hook/function combination that can be added to my theme's functions.php to properly disable REVISIONS and AUTOSAVE for the entire wordpress installation? What about if just for a certain custom post type? Searching online gives various hacks from deregistering scripts to tampering with core files. What's the acceptable/correct way to do this?