There's a lot to be said for well-crafted vision and mission statements.   I've been researching and leaving a trail at The Bookshare.

In a Nutshell

• Mission - who are you? what do you do?
• Vision - where do you want to go?
• Values - what do you value? what's important? (your corporate culture)

How Do You Craft Them

1. You start by figuring out the values.  You figure out the values by observing how your organization prioritizes and how they spend their time.  There can be a gap between what folks say they value and what they actually do.  Actions speak louder than words.
2. Once you know your culture and values, you can figure out your mission -- who you are and what you do.  What is your organization's unique value you bring to the table?  What is your unique strength?  In a world of survival of the fittest, this is important to know and to leverage.
3. Now that you know who you are, you can figure out where you want to go.

A good vision statement is a one-liner statement you can repeat in the halls.  Nobody has to memorize it.  It's easy to say and it's easy to groc.  The same goes for a mission statement.  You might need to add another line or two to your mission statement to disambiguate, but if folks don't quickly get what you do from your mission statement -- it's not working.

How Do You Use Them

• Use a mission statement to quickly tell others what you do.
• Use a vision statement to inspire and rally the team.  It should be on the horizon, but achievable and believable.
• Use a mission statement as a gauge for success. 
• Set goals and objectives that tell you whether you're accomplishing your mission and moving toward or away from your vision.
• Use your mission to remind you what you do (and what you don't) and to help you prioritize.
• Craft a personal mission and vision statement to help you get clarity on what you want to accomplish.
• Use your personal vision and mission statements to help you stay on your horse, or get back on, when you get knocked down, or lose your way.

Examples
I'm a fan of using reference examples (lots of them) to get a sense of what works and what doesn't.  The Man on a Mission blog is dedicated to mission statements and has plenty of real-life examples to walk through.