"What convinces is conviction.
Believe in the argument you're advancing.
If you don't, you're as good as dead.
The other person will sense that something isn't there,
and no chain of reasoning, no matter how logical
or elegant or brilliant, will win your case for you."
-- Lyndon B Johnson
Believing in what you do is the single most important quality and your belief shows in many ways even without speaking it.
When I was first learning about sales and sales management in the auto business in the 1970s, I listened to tapes and read books from Zig Ziglar. I'll never forget as we was telling how he advised a salesperson in how to succeed in the company he was involved in. He sold cookware. He asked the salesperson if he owned the cookware and when he answered no, Zig told him that without owning and using the product, convincing others would be a serious challenge.
When I started in the auto business, I made sure to own the product as soon as I could, and I think it made a huge difference. As a sales manager, I talked about that often to my staff. A lot of car sales people would think that because of their knowledge of the business that it would be in their best interests to own a used vehicle and avoid the initial depreciation of a new vehicle, but if they never bought a new vehicle they wouldn't be able to relate to that special feeling that only comes from buying a brand new automobile. While I was in the auto business I bought about 15 brand new vehicles and a couple of used ones. I wanted to make sure that I led by example, but more than this, I loved that feeling of owning a brand new vehicle and so it was very easy for me to sell a new vehicle to others.
I've seen a sales person work at a Chevy dealership, making their living selling new and used vehicles. Then, I would see them go out and buy a brand new Honda. That act speaks to their conviction, or lack of it. What does that say to potential clients if they found that out? In this example, the best thing to do for this person is to go sell Honda products. This way their conviction will match their behavior.
Nothing convinces like conviction.