Preview Flattening In Photoshop CS4/5
You think you are done but you know from past experience that sometimes when you flattened you see something that needs changing. You can Ctrl Z or use the history panel BUT it can be an endless cycle (ctrl Z, make a change, flatten again, find something else, ctrl Z, make a change …). However if you press Ctrl/Cmd + Alt/Option + Shift + E (it is easier than it sounds), all visible layers will be merged into a new layer. Now you can preview the flattened image or even experiment with further changes to a flattened image and just delete the layer when you are done.
Easy Peasy Louisey.