Here is a post I made a while back that is now doing the rounds on the article circuit... worthwhile reading...
If you feel the need to offer proof -
Is it possible that you don't really believe that the product you're selling actually can do what you say it can do in the hands of others?
Or is it that you just know that your sales copy is crap and people won't buy it without "proof".
Maybe it's that (on the other side...