How to have lasting relationship with clients?

Clients are the most precious assets for a business. Without clients, there can be no business. With poor quality of clients, the business will be poor and if you manage to get very good clients and retain their loyalty, your business will only go up and up. This all sounds very exciting. But it is not easy to get very good clients and all the more difficult to retain them. After all, whatever you do, your competition is trying the same and may use better techniques to get business. Are there any innovative approaches to client relationships?
We are talking about direct sales in this discussion and not about selling merchandise to large consumer base. For example if you are a contractor maintaining air conditioners in clients work places. Or a direct seller of computer hardware to business buyers, and all such businesses where your sales to individual clients are large, and you are in direct contact with clients.
The first need is of course client satisfaction. If the client is satisfied with your response time, after sales service and can depend on you, pricing may become secondary. All clients do not buy from a supplier whose sales at the lowest price. If your product cost is a small percentage of clients total expense or if your product is essential for your clients, you are onto something good. How to retain such clients despite all the competition? What are the other factors than client satisfaction?
Relationship is one such other major factor. Do you relate with your clients only professionally, or are very good friends? Both these extremes can hurt. For a long-term business relationship, good friendship is not good for health of your business. Any problem in the personal friendship will directly affect your business. What if you relate to your clients mechanically in a professional style totally devoid of personal touch? You know the answer yourself.
What is needed is a relationship that does not border on personal friendships, but crosses mechanical approach. A fine balance between personal and professional.