Writing down your ideas so you don't forget them can be sensible. But as photographer Chase Jarvis reminds us, sometimes it makes more sense to go ahead and just do something rather than putting in the never-never on some unspecified future to-do list.
Obviously this won't work with every idea you have; some concepts need time to percolate, and there are only 24 hours in the day, some of which you need for sleep. But there can be something exhilarating about just diving in and starting something, not holding off until what seems like the best time.
The next time you have a great idea [Chase Jarvis]