Any project has a certain amount of planning that's required before a single line of code can be written. As I've learned more about the craft, the scope and complexity of my personal projects have increased to the point where my usual of M.O. of "scribbles in a legal pad" isn't going to cut it anymore. What are some tools that you've found helpful in organizing your thoughts around a new project, either on your own or with a team? Are UML-style diagrams or prototypes a must, regardless of project?

When a brand new brilliant idea hits, what's step one towards realizing the goal?