"Most corporate mission statements are worthless. They consist largely of pious platitudes...A mission statement should not commit a firm to what it must do to survive but to what it chooses to do in order to thrive. Nor should it be filled with operationally meaningless superlatives such as biggest, best, optimum, and maximum..."To test for the appropriateness of an assertion in a mission statement, determine whether it can be disagreed with reasonably. If not, it should be excluded. Can you imagine any company disagreeing with the objective "to provide the best value for the money." If you can't, it's not worth saying.
"What characteristics should a mission statement have? First it should contain a formulation of the firm's objectives that enables progress toward them to be measured....Second, a company's mission statement should differentiate it from other companies...Third, a mission statement should define the business that the company wants to be in, not necessarily is in. However diverse its current business, it should try to find a unifying concept that enlarges its view of itself and brings it into focus...
Fourth, a mission statement should be relevant to all the firm's stakeholders. These include its customers, suppliers, the public, shareholders, and employees. The mission should state how the company intends to serve each of them...Finally, and of greatest importance, a mission statement should be exciting and inspiring. It should motivate all those whose participation in its pursuit is sought...
"If your firm has a mission statement, test it against these five criteria. If it fails to meet any of them, it should be redone.
Read more in this paper by Russel Ackoff. You may also wish to check out these related posts.