Well, closing a sale isn't just about closing the sale. A number of things should take place beforehand to ensure that the consumer knows all they need to know about the product in order to confidently purchase it.
You need to initially create a relationship with the consumer to earn their trust. If you spend time investigating what it is they need, then you'll be better able to explain how your product can meet their needs or solve a problem. Demonstrate all of the features and benefits of the product with confidence and knowledge. Be prepared to handle any objection they throw at you so that they understand exactly how the product is going to help them.
Then look for clear indications that they're interested in buying. Offer different purchase options and customize the closing of the sale if need be. Don't get too pushy at the end of the sales process if they still seem hesitant. Best to give them a summary of the product benefits, hand them your card and follow up during the following few days.
At the end of the day, your mentality around "selling" should be more of a consultation/problem solving approach. You are helping someone else solve a problem using your product or service-- Your job is to simply show them that you have a solution, rather than to "sell" them on your business.