I found this great tutorial full of wonderful tips for making realistic shadows on your scrapbook layouts with Photoshop... Also by Megan Turnidge with a set of layer styles included.
eta: realistic cluster shadows tutorial (12/11/12)
I found this video tutorial for realistic shadowing of clusters on Kristin Cronin-Barrow's blog, done by Christy Haig here.
eta: 5 more realistic shadows tutorials (12/13/12)
Here is the Sweet Shoppe link to several more awesome shadowing tutorials including: Basic Shadowing Techniques: Layer Styles, Basic Shadowing Techniques: Anatomy of a Page, Shadowing Acrylic and Plastic, Lighten up, baby!, and Using the Distort>Wave Feature on Shadows in Photoshop [enJoy!]