This is an interesting feature that we all should get in the habit of using. If you open your History palette in Photoshop you will notice that as you work, things stack up fast. With only a limited number of states your History can handle (by default I believe it should be 20) it would be a good idea to start taking Snapshots of your work.
When I say this I don’t mean screenshots. Actually if you look at your History palette, at the bottom there is a small button that looks like a camera. This is the Create New Snapshot button. When you click it, at the top of your History palette it will add a snapshot that you can select later on if you don’t like the direction your image is going.
For example, if you are working hard to create an image that suggests how difficult life is in a wartorn country, but then get off on this kick of lightening the eyes and teeth, fixing the hair and removing blemishes from peoples’ faces… it’s easy to revert to how it was before if you have done a snapshot.
Unfortunately the snapshots won’t save with your file, so if you’ve already saved and closed your document then you are pretty much stuck with how it is, but it’s a great trick if you have a lot going on in your image and need to make sure you can get back to how it was before.