Adobe Illustrator CS5 Tip - Masking reversed
Instead of creating complex artwork and then drawing another object on top to mask that artwork, you can now simply click on any object and change the drawing mode to Draw Inside (Shift-D) at the bottom of the Toolbox. After you’ve set Draw Inside, the selected object will mask everything you draw. Draw Inside also supports the Paste and Place commands-pasting inside an object has finally come to Illustrator.