The best way to communicate with an existing customer base is through a combination of e-mail, direct mail, and one-to-one phone or in person contact. This is superior to relying on a single form of communication, such as letters alone, because it will help keep your recipients engaged when you mix things up a bit. I don't know the type of business you're in or what kinds of offers you are considering, but for many businesses it's best to vary the tone of the message. In other words, some companies might alternate sending direct-mail solicitations with an online e-mail newsletter or other soft-sell company information, such as success stories. You should also consider creating a customer reward program that you can promote via e-mail. Best wishes for success!