In his book called "How to Win at the Sport of Business," Mark Cuban makes an important point that your customers do not know what they don't know. 

If you are a Business Intelligence consultant, this means that your clients may give you the worst advice on how to build a BI application. They may understand their business problems, but not how to solve them with BI. Instead, you as an expert need to tell them how things should be done. 

Of course, if they are to listen to your advice, they must first trust you. Building trust on your BI engagements is one of the most important things you can do. David Maister once wrote,
"The key point is that trust must be earned and deserved. You must do something to give the other people the evidence on which they can base their decision on whether to trust you. You must be willing to give in order to get." 

You cannot demand trust; instead, you must work hard at developing it. Until you have the client's trust, they will more than likely force you to follow their own mistaken approaches instead of listening to your good advice. 

See this Entrepreneur magazine article for Mark's comments.