Take snapshots of your work for precautionary measures (Photoshop CS2/CS3/CS4/CS5)

When you're working on an extensive project in Photoshop, it's always a good idea to take a few snapshots as you go. That way, if you want to backtrack or something goes awry, you can click on a snapshot and get to work again. To create a snapshot, open the History panel and click the Create New Snapshot button at the base of the panel. Upon doing so, Photoshop places a saved version of your file in the History panel. Then, if you need to access that image, just click on it.