Bowl them over with your service. Become such an integral part of their companies that you have your own desk in their offices. Develop solutions before they even discover a need.
You won't have a lifelong relationship with many customers, but when you do, you'll profit from it. I have a handful of customers with whom I've done business for many years at more than one company, including my own. A few represent millions of dollars in income over the years. You can enjoy the same kind of long-term relationship with your best customers if you never stop selling them.
They may become your friends. They may rely on your service or products to the exclusion of all others. They may tell you that they'll always be your customers and sign long-term contracts to prove it. But if you take them and their business for granted, you'll regret it someday.
Excerpted from Creative Selling: Boost your B2B sales