Adobe Photoshop CS4 Keys for using the History panel keyboard shortcuts keys

Shortcut                                                                          Command
Optn + New Snapshot	Create a new snapshot
Double-click snapshot name	Rename snapshot
Cmnd + Shft + Z	Step forward through image states
Cmnd + Optn + Z	Step backward through image states
Optn-click the image state	Duplicate any image state, except the current state
Optn + Clear History (in History panel pop*'up menu)	Permanently clear history (no Undo)