If you'd prefer to keep more memory available for Photoshop, it's a good idea to get in the habit of creating snapshots of your image at key points throughout the editing process. That way you can still revert to an earlier state without having to dedicate more memory to keeping additional history states.
While you're in the Photoshop Preferences, take a look at the bottom section of the preferences dialog labeled History Log. Introduced in Photoshop CS, this feature offers a wonderful way to keep track of your editing history on an image as you work with Photoshop. By default this option is disabled, but you can learn how to set it up in my article, Keep Track of Editing History in Photoshop.
