Whenever you save, save two copies. Call them 1 and 2, or A and B. Whichever you save to second is the one autosave will overwrite. You'll have the other one as a backup of pre-autosave state. That way if the game autosaves something you don't want saved, you can restore the other version. But then, make sure to re-save that version.
Example:
Save to Save1 and Save 2. I lose a game, but only because a bat flew in my window and attacked me. After the game the autosave saves to Save 2. I re-load Save 1 to go back to pre-autosave. Before doing anything, I save back to Save 2, so I again have two copies of the desired state saved. Autosave will again save to Save 2.