Tell the absolute truth about your brand and your product.
Or even your category and its consumers.
Be at pains to point out imperfections and shortcomings.
Admit that mistakes were made and promises were broken.
Apologize for having failed to provide absolute satisfaction.
This approach can be very disarming because consumers expect advertising to be full of boast.
So when you admit to your weaknesses - as an individual or even as a company - it can be refreshing.
It will also make you more credible when you do come around to pointing out your strengths.