DR Master Joseph Sugarman once used the analogy of a "slippery slide" to describe how good advertising copy should flow. I always remember that analogy and try to apply it to every advertisement I work on. Below is his original description, an excerpt from his book, Advertising Secrets of the Written Word:
Picture a steep slide at a playground. Now picture somebody putting baby oil or grease along the entire length of the slide including the side rails. Picture yourself now climbing up the ladder, sitting at the top of the slide and then letting gravity force you down the slide.This advice is just as valid when applied to a commercial, with the "problem opening" filling in for the headline and the product demonstrations representing the body copy and so on, each one so compelling you must continue watching all the way to the blue screen.
As you start to slide down and build momentum, you try holding on to the sides to stop, but you can't stop. You continue to slide down the slide despite all your efforts to prevent your descent. This is the way your copy must flow.
Every element in an advertisement must cause that slippery slide effect. The headline must be so powerful and compelling that you must read the subheadline, and the subheadline must be so powerful that you are compelled to read the first sentence, and the first sentence must be so easy to read and so compelling that you must read the next sentence and so on, straight through the entire copy to the end.