When most people think of a mission statement they think a of some stodgy cluster of bullet points that state trite company values and objectives.  When done poorly, these appear designed to falsely boost employee morale, pepper press releases and fill-in white space on marketing collateral.  Unfortunately, more often that not mission statements are shams and they do very little to help the company.  However, when done well they are tremedously valuable.

A mission statement is a single phrase or sentence that encapsulates your company's unique objective. 

Unique is a key word here.  Mission statements are most effective when they focus on the unique solution that your company intends to provide.  There are two reasons for this.  First, including commonplace objectives such as becoming a large company or generating profits does not clarify what makes your company different.  Second, they do not implicitly communicate the pain point that you intend to address. 

Google has a great mission statement, "Google's mission is to organize the world's information and make it universally accessible and useful."  It's clear what they are trying to achieve and the pain point is obvious: the world information is currently highly inaccessible and useful.  Big visions are the foundations of disruptive companies, so when someone hears your mission statement they should be able to understand what about the world you are going to change.

It's also critical that the mission statement focus on the 'what', not the 'how'.  eBay's mission is "to provide a global trading platform where practically anyone can trade practically anything."  In this one sentence it's not critical to understand that they will leverage technology, hire marketing people or otherwise in order to do this. 

Some companies confuse mottos and mission statements.  Motto's are a summary of values that management hopes to superimpose on their staff.  In my opinion mottos generally come off as disingenuous.  Here's a cheesy one: "Respect, Integrity, Communication and Excellence."  That was Enron's.

In sum, mission statements need to be short, unique and focused on the 'what'.