If you have a unique product that does something other competing products cannot do, demonstrate it.
If you can't demonstrate it, bring out a sampler.
If you can't bring out samples, encourage your customers to talk about it.
If your business is new, get someone with authority or repute to vouch for it.
If none of that is possible, at least demonstrate (show, don't tell) that your product is in high demand.
If all you can do is talk your own products up, you really have no proof.
Nothing converts prospects into customers/clients as well as proof.
And nobody wants to go to a nightclub that doesn't have a long queue outside.