If you feel you have to "persuade" someone to buy your product or service, you probably haven't been effective in demonstrating how it will help them. Once again, in order to do this, you need to learn what is important to the other person. Once you have this information, you can adapt your presentation to address their specific issues or concerns.
As Jolly Roger pointed out, ask questions. My experience has taught me that most people will give you all the information you need to help them make an educated buying decision. And, many people will persuade themselves providing they have the opportunity to talk and express their objectives, concerns, wants, needs, likes & dislikes.
Cheers!
Kelley