Scoble's essay on persuading is right on. I'm persuaded.

People won't be convinced to buy your stuff by arguments based on your competitor's stuff being crap. You'll just look like another clueless salesman.

Being an authority in your area, and making it clear that you're working for the customer's interests, makes all the difference. And it's a nice way to build enjoyable and long-term relationships, which make work much more fun!

For me this is one of these I knew it, just couldn't put words on it reads. Definitely recommended.