Similar to jhockings answer but a little quicker: You can always right-click on the layer(s) you want to copy and select “duplicate layer.” From there you get a dialog box and you can select the destination of the layer. The current document is selected by default but you can change it to an open document or new document.
If you want to put the duplicated layer in a specific location in an open document, make sure to select the folder you would like to put it in before you duplicate the layer. You do this by literally selecting the desired folder in your layers palette for the destination document.
If you find you need to repeat this action many times, you can always make a new action for it. Open the Actions Palette and create a new action – set your hot key (I use F2). Then, to record the action, click the Options button (upper right corner of the palette window) and choose Insert Menu Item. Select Layer>Dupicate Layer. Then click Okay. Now, when you hit F2 while a layer is active, it will bring up the duplicate dialog box. Pick your destination and you’re done! Just be careful when choosing hot buttons because it can override some of the default actions of photoshop.
