If a plot is stagnating a little, the car is a good place to revive it. Maybe it's just the changing scenery or the drone of the engine, but something appears to unleash the creative flow quite well. The only snag is remembering it afterwards. Fortunately, my car trips tend to be short local ones and, when I stop, I write things down. It's a good excuse to go into Costa and have a lovely drink at the same time.
So this is why it is always a good idea to have a notepad and pencil. I have also got a dictaphone, but nothing beats the scribbled word on a page.