If you hate what you just did in Photoshop, hit the usual ctrl (command on a Mac) + Z to undo it . And if you hate what you did a few steps back, go to the history Panel and click on the last step you liked. But, by default, Photoshop stops keeping track after 20. To gain more access to your history in CS3 or CS4, go to Preferences > Performance. Then choose to save up to 1,000 history states. Don’t forget: You’ll lose the list when you close the image.