I've had some success with: 1) duplicating the layer and moving it under the object then nudging it in the direction the shadow should fall, 2) adding a layer style of Color with black, 3) duplicating that layer several times and moving each one further away from object, 4) blurring each layer (Blur->Gaussian) a little more strongly based on how far away it is from the object, 5) merging all of the layers together and then changing its blend mode to Multiply and playing with the opacity until I'm happy. How far you move the shadow layers, how much you blur, how many layers, etc. is all highly dependent upon the height of the object. You can see the technique on my Argria map in the CWBP (link). Probably not the best method but it has worked semi-well for me in the past. Hope that helps!
M