The holy grail is the "Create Layer" menu item - it's the cure to the 120° disease.
Fire up PS
Create your object
Apply a boring drop shadow in Blending Options (right click on layer and select via the menu), but in the dialog make one little change: set the angle to 90° http://screencast.com/t/7VtB2W4skwu
In the layers menu, right click on the "Drop Shadow" effect of your boringly shadowed object, and select Create Layer. This will effectively remove your drop shadow as an attached, non-destructive effects layer into its own rasterized image layer. * NOTE: choosing this option will rasterize ALL of your blending option effects, so duplicate the layer for non-destructive awesomeness. *
Now, you can select the layer, and transform the heck out of it. I opted for the probably overused but still cool "lifted page" style by messing around with the Warp setting of Free Transform http://screencast.com/t/E02xjDm0YP
Lots of little embellishments can happen after this stage, but that's the quick 15 second way to make it look like the only light source in your document comes from the northwest.