Best practice would arguably be to Duplicate the document the moment you open it, then proceed with your edits. However, sometimes we open a document and start working, only to realize that it's now (in our mind) a "new" document based on the original.
Applications that support Lion's versioning will give you the option to Duplicate and Revert, meaning that they will make a copy of the current document in it's current state, then revert the original document back to its last saved state.
If some unwanted changes have been saved within the original, you can use Revert to restore the original to the state you prefer.
If a document hasn't been opened for a period of time, it will open in a locked state and you will be prompted to either unlock or duplicate it before you can make changes.