This video gives a ten minute tutorial on how to achieve the superimposition effect for web videos with a little props and software.
You will need to get Wax, a free nonlinear editing system for Windows that includes the chroma key effect. You will also want to get the codecs HuffYUV, Lagarith, and MSU Lossless. Then there's styrofoam, green board, a yard stick, an ugly seventies lamp, a compact fluorescent light bulb, a clamp lamp, plastic vase, books, and a cardboard box. And last but not least, a camera.
See how you can make the perfect green screen environment for your YouTube videos.