Shadow and other fancy effects and non essential graphics should where ever possible be kept seperate from the content. Ideally a good design is if you disabled the style sheet you would see a well structured plain text document.
so with that in mind, if say you want a shadow effect on both or one sides of your content, and assuming your content container shadow is going under is a fixed width, then the simplest method for both the aforementioned is to create an image how you want your shadow to look, then crop out thin strip, say 10 pixels high then repeat this as a vertical background image on the container you wish to apply it.
if you want a bottom shadow then you will have to create another div container and place it immediatly beneth the content container you wish to have the shadow. in the CSS you will have to specify a height for this shadow container or nothing will display in some browsers., then simply repeat the above process only horizontally.
This process is also a technique used to create the illusion of having equal length multiple coloumns.
with css and multiple nested divs you can get very creative, just be bold and try somthing, if it works , it works, if not try again. oh and make sure your code is valid! if somthing dosnt work properly then its often a typo somwhere.