Applying Layer Styles Multiple Times

Usually, layer styles can be applied only once. For example, if you click on Layer → Layer Style → Drop Shadow, you cannot create a double drop shadow, one of which has an angle of 120°, a distance of 2 pixels and a size of 2 pixels, and the other of which has an angle of 180°, a distance of 12 pixels and a size of 12 pixels.
Actually, it is possible! It just requires a little detour. Create the first drop shadow as you normally would. Then right-click on the layer and select “Convert to Smart Object” from the menu. This smart object can be assigned another drop shadow, and you can convert the smart object into yet another smart object. This way, you can easily add a third and fourth drop shadow. Alternatively, you could apply multiple strokes.

By the way, to put one or even several styles onto their own layers at once, right-click on the FX symbol and select “Create Layer” from the list. Now you can apply filters to these styles, but they won’t be editable anymore.