Now some people may say this is really a sales tip or even a customer service tip. As a marketing professional who also directs business development (aka sales) and  client services (aka customer service) I can tell you this: it’s all marketing!

Call back when you say you’ll call. Reply to emails on a time everytime. Provide additional information when you’ve promised it. Ask how you can offer even more than you already have, and then do it. Every contact point is an opportunity to market your products and services. Every contact point is a new chance to solidify the relationship.
Differentiate yourself from all those other companies who are begging for  your customer’s business. Give more every time.

Do it with a smile on your face and in your voice.