Having tried writing a mission statement a few times, it can be one of those things that gets trickier the more that you think about it, I find. It can be helpful to separate it out into mission and vision though — a vision is an external state of affairs, where you can be all sci-fi dreamer about the world and let yourself be all imaginative. “A world in which everyone feels free to dress up as whichever cat they want to.”
The mission then is just a big project description ;-) But starts to make things more practical, and is the link between what your vision is, and what you’re really doing, day-to-day. “Our mission is to discover and bring back new forms of reearch”, or “our mission is to eat as much pizza as we can”, for exaple. Note that neither of these do anything for people who want to dress up as cats but feel like society would ostracise them.
Here are our current vision and mission: http://ocsi.uk/values/ (they shouldn’t change much without a lot of thought, but you can always reword them to make them clearer.) But I like the idea of having a “team” vision and mission — have been thinking the same for values for a while. Maybe I need to make that happen…