A few weeks ago, Computer Arts magazine published a PDF tutorial about various clipping and opacity masking techniques in Adobe Illustrator.
"Ben the Illustrator shows you the range of brilliant features hidden within your masking tools.
A mask might appear to be just a simple tool for covering something up, but Adobe Illustrator’s masking tools offer far more than one basic use. Along with the Compound Path, Illustrator’s Clipping and Opacity Masks can take your illustration and design onto a new level of detail, precision, texture and shape. To know when to use each tool, you simply need a clear understanding of what each one does." Full tutorial & source files.