Learn the art of looking into someone's eyes. Looking straight into the customer's eyes has a very powerful effect in their decision as to whether they would buy your product or not. However, you should keep the eye contact to a moderate intensity or else you might be misinterpreted or you might get in trouble for doing so. Don't underestimate the power of these covert persuasion techniques. They may be simple, yet they have been proven to work time and time again.