This is a big mistake. Your present customers are your best source of new business and the easiest group to reach. You know who they are. They like doing business with you and have already been sold on your company. Take some time to think about what you can do to increase your sales to your customer base.
Many businesses send out newsletters to their customers as a way of keeping in touch.
Copy what the big guys are doing. You can send special promotions along with your invoices as a way to add business. Let your customers know that you appreciate their business and they will remain loyal, satisfied users of your products and services. They had a product that everyone bought but it lasted a very long time and repeat orders were slow. When their marketing people put it in a box and sold it as a deodorant, sales soared.