As Kristian has pointed out, good control over z-values will often solve your problems.
If that does not work you can take a look at CSS Box Shadow Bottom Only on using overflow hidden to hide excess shadow.
I would also have in mind that the box-shadow property can accept a comma-separated list of shadows like this:
box-shadow: 0px 10px 5px #888, 0px -10px 5px #888;
This will give you some control over the "amount" of shadow in each direction.
Have a look at http://www.css3.info/preview/box-shadow/ for more information about box-shadow.
Hope this was what you were looking for!