While skimming through Harry Beckwith's marketing book The Invisible Touch, I came across this section which describes 8 keys to lasting relationships (with your customers, that is). They are:

1. Natural Affinity.
2. Trust:
3. Speed.
4. Apparent Expertise.
5. Sacrifice.
6. Completeness.
7. Magic Words.
8. Passion.

To put it bluntly, seek clients you would want as friends.