1. Point out the strengths of the new system. Do so in a way that contrasts with D&D without attacking D&D. Often pointing out that the design goals are just the opposites of the other, without either being "better".
2. I almost always try for a very different genre. Otherwise a lot of people will look for the usual mechanics, and they may not exist.
3. Pitch it as a one-shot or very short campaign.
4. Honestly, almost pitch it as a different type of game. It's not "D&D with different rules." Point out what kinds of things it would do, and use analogies like "it's like going for sushi instead of steak". Both are good, but just different.
5. Don't. Attack. D&D.