The key thing to focus on is the imbalance of power SOPA creates and how that will end up getting in the way of Uncle Art's evening entertainment. So far the shoplifting shutting down the store analogy is the most pointed I've heard, but still not a catch phrase.
My hope with the term 'Prohibition' is to turn it around and put the focus on the evil being done by the media companies. They want the government take away your rights, because they don't want to compete fair and square with web technologies.
Here's an analogy I've been using that might work: "They're tying your hands behind your back so you have to pay them to feed you."