If you're looking for new business, you may be ignoring one of your best potential sources: old customers. That's what Michigan-based Van Pelt Corp., a steel service center, discovered last summer. During a period of sluggish sales, one of the company's inside salesmen sifted through its computer files and came across hundreds of customers who hadn't placed orders in more than a year. He began phoning them and, in a few weeks, generated more than $25,000 a month in additional sales. When salesmen left the company, for example, their accounts often were not pursued as aggressively as they might have been. Now, the company calls old customers regularly as part of its sales program.