If there are tangible or even measurable results in your product or service, you should show those metrics and results. What you say in your marketing messages must meet the expectations of customers, who will have to get those results themselves. They will therefore rely on what you can demonstrate beyond any marketing mumbo-jumbo.
Try to collect success stories and testimonials from satisfied customers that you can share with your audience, asking them for permission to share their opinion or their case. The cases you use should highlight how your product or service has generated positive results for others, so that your audience internalizes that it can do the same for them.
Use real, empirical data, accompanied if necessary by statistics to back up your claims about the results of your offer, and don’t create false expectations. The more evidence you can provide of the effectiveness of your product or service, the more persuasive your message will be and the more likely it will be that those it reaches can be converted into customers, making your marketing campaign truly effective and providing you with a worthwhile return on investment.