Communicate
Building a lasting relationship requires communication. Communication means listening to your customer or client, as well as conveying your own message. One sided communication is a speech, mutual communication is a conversation. Communicate when things are going well and when there are problems. When things are going as expected, bring it to their attention. When there are problems, provide the scope of the challenge and your solutions, preferably before the customer even knows that the problems exists. This builds credibility, confidence and trust.