The Fun Extras - Set Six
119. Undo History (link to comment)
Are you addicted to those undo and redo buttons? Take it a step, or shall we say, state, further and use the undo history palette.
In the full version of photoshop, you can also create snapshots through the flyout menu.
When you do this, however, it does "clear" all your states. It's a great way to preserve a certain point to return back to. You can choose the original snapshot state created when you open the document or any snapshot you manually choose to create.