Have you tried email marketing? It is a great way to reach your customers (and potential customers) with information that can help to keep them engaged with your business.
Let them know about special offers, new products, trade show schedules, office hours, etc.
The cost for this service is surprisingly low, and the efficacy is very good.
Just use the contact form and request a free two-week trial. After that, the cost depends on the number of messages you send.