Some of my fellow bloggers on the LadyKillers blog (http://www.theladykillers.typepad.com/) recently discussed the phrase, "The one that got away." In a large sense, missed opportunities are the escapees in question. You meet a great guy at the airport while waiting for a connecting flight, have a few laughs, share some stories, but then never see him again. The perfect job vacancy opens up, close to where you live no less, but the spot goes to a more experienced candidate. A relative offers you her cruise tickets when she's unable to go herself, but you can't get the time off from work and have to pass. All those situations leave you wondering what could have happened had things gone differently.
In terms of my writing, the one that got away is shrunk down to a much smaller scale and refers to all those great phrases and bits of dialogue that pop into my head in the middle of the night, or while outside with the kids, or during a long drive, that then vanish before I can write them down. Often, I'll wake in the wee hours of the morning and create entire scenes in my mind. At the time, the action and dialogue seem so vivid that I can't imagine ever forgetting them. But I do. Time and time again.
So now I use notepads to catch all those fleeting thoughts. I started by placing a pad and pen on my nightstand. But then my kids thought it was their own personal drawing set. Oh, the giggles as they merrily scribbled over my midnight chicken scratch. Now the notepad sits on my bathroom counter, behind a baby gate, far enough back that my oldest can't easily reach it. There, I jot down the main points for the next chapter, or recurring themes I want to keep in my mind as I write, or a clue that I need to weave into the plot. It's not a perfect system. Often times, I'll think of an idea while I'm downstairs or outside, and then have to hope I'll remember it by the time I get upstairs again. But I tried multiple notepads around the house and found it was a hassle keeping everything together, especially since I sometimes lost the notepads altogether.
So one notepad is the best plan. Just the process of writing down the idea frees my mind to come up with new ideas. And after a while, I have an entire book. Then I can start the whole process over again.