Flattening all of your layers in Photoshop is usually bad. But sometimes I just want one composite layer that has all my changes on it to work from and I don't want to duplicate the document. There's an easier way.
Click once the top layer in the Layers palette. Then press this (incredibly complex) keyboard shortcut: Command-Option-Shift-E (PC: Ctrl-Alt-Shift-E). Photoshop will merge all of the layers into one new layer and insert it at the top of your layers stack. It also leaves all of the layers below alone so you can still go back to them if you need.
(For more on Photoshop and layers, be sure to check out my latest book, Layers, The Complete Guide to Photoshop's Most Powerful Feature.