Pile It High: Coat the top of the box with spray adhesive (available at craft stores), then sprinkle on Styrofoam packing peanuts (pick some up at office-supply stores). Cover those peanuts with more spray adhesive and add more peanuts on top. Repeat until you have a mound of fluffy popcorn.
Earn Your Stripes: Create the iconic look with red gaffer tape (sold at craft stores) applied two inches apart.
“Pop!” Goes the Type: Red felt letters (from a craft store or cut from felt with a stencil) really stand out when you give them a white felt background. Attach them to the box with spray adhesive.