How do we develop or revise our organisation’s mission statement?

Our mission statement is about fifteen years old and it really doesn’t reflect what we do anymore. We started as a youth sports programme but now we do much broader community development work. The board has been trying to revise it but every time we try we end up in a long debate and can’t agree on wording. Is there a structured process for revising a mission statement that might help us get to something everyone can actually live with? And who should be involved — just the board or wider stakeholders too?