When you're selling something your customers will probably ask for something, whether it is information about the product, lower price, a product demonstration or customer referrals. A good rule to remember is to ask for something in return: for example, if the customer asks for a demonstration, ask them for a commitment to move forward with the purchase, if the demonstration proves that your product or service fulfill their needs and wishes.
Don't give in to temptation in promising something that you can't deliver. If a product needs time to be shipped, don't tell your client that you can get the product right away. Be honest and realistic.