Every object will want a drop shadow for realistic effect. But the standard drop shadow will dull most transparent objects, even those with some color. There is an easy way to apply the drop shadow and yet not lose clarity of the object. Below for our example is the glass star from An Elegant Christmas page kit.
Apply your drop shadow as usual, using Effects, 3-D Effects, Drop Shadow. But make sure to check the "Shadow on New Layer" box.
With the shadow fully there, you see how it dulls the effect of the glass, as the grey runs underneath the object.
Now you will want to use your wand tool. With this you'll go to the layer of the element itself and select all opaque. Now switch back to your shadow layer, and Edit, Cut. This will remove all the shadow underneath the object, while still leaving the shadow dropped to the side where you need to see it.
And now our star shines again, but with the realism of the shadow!