I have been in Sales and dealing with clients now for well over 20 years and IMHO the following rules have worked out pretty well for me:
1. Be honest with your clients, if you don't know an answer, explain that you will research the question to find the answer.
2. Treat even the guy sweeping the floor today as if he may be the boss tomorrow, really nothing more than that old Golden Rule, it really works
3. Care about your clients helping them to prosper will make you prosper
4. Form strong lines of communications. Even when times are bad if you have good communications it can really go along way
They are all pretty easy to figure out and while I may not have won every deal I have formed some very good long lasting relationships with businesses that know they can trust me long-term
Bottom line just treat everyone as if they are important, isn't that ultimately your expectation that you and your business are important and priority #1?
