The problem is that many of us have been burned by waterfall so badly that we take a "waterfall sucks and agile doesn't" approach. Unfortunately this approach doesn't work because it just leaves a bad taste in people's mouths and we then justify it with the concept that you have to experience agile to truly get it.
I've been following Seth Godin lately since he is a master marketing guru and his viewpoint provides insight into how to sell a new idea. Today he wrote a great post about how to pitch a concept. He uses the metaphors of gravity vs. evolution and how quickly these concepts were "sold". Here's a snippet:
Tactic 1: Try to tell a story that complements an existing story rather than calling it out as false.This is why I like Mike Cottmeyer's recent work around bridging the traditional PM's point of view with Agile's (he has a bunch of posts, so keep looking past this recent one if you are interested). Instead of saying waterfall stinks, he explains how agile enhances.
Tactic 2: Try to make the 'proof' as vivid and immediate as possible. Like an apple falling on your head.