Let's suppose you've been working on an image, done some Cloning and Healing, adjustments of Levels and Colour and you've got quite a number of Layers. You now want to experiment with some further creative work, but don't want to lose your work to date if it doesn't work out.
1) If your Layers Palette is 'docked' in the top right box on your screen, drag it out and down
2) In the Layers Palette, click on the New Layer icon (or Layer - New - Layer, or Shift Ctrl N)
3a) Now for the tricky bit: holding down the Alt key, move your mouse over the little right-arrow at the top of the Layers Palette, click and hold down the left mouse button (you are still pressing the Alt key aren't you?) and drag down to highlight Merge Visible, then let go the mouse and let go Alt.
3b) Alternatively: holding down the Alt key, hold down the left mouse button and click on Layers in the menu at the top of your screen (you are still pressing the Alt key aren't you?) and drag down to highlight Merge Visible, then let go the mouse and let go Alt.
4) Double-click this new layer's name and call it what you will (test? experiment? copy to date?)
5) Now make your further adjustments. If you like them, fine. If not, drag this new layer down into the rubbish bin, and start again.