Create drafts of WordPress posts/pages/CPTs even after they've been published
Go to a post's edit screen and click on the "Save a Draft" button in the Drafts of Revisions postbox (make sure you've enabled the post type first). The post's core data, taxonomies, and meta data are all copied into a new post - the draft - as a child of the original post. The draft has a custom post status; it will never show up in any queries for posts. You can create as many drafts as you like.
You can edit a draft's post content, taxonomies, and meta data as you like and save progress with the native WP "Save Draft" button. You can also preview the draft and compare changes against the parent at any time (even comparing changes in taxonomies and meta data).
When you're ready to update the parent post, click the Publish button from the draft's edit page. All post data, taxonomies, and meta data are merged back into the parent post and the draft post is deleted.