Helps to keep a notepad and pen with you at all times. Seriously. Because even flicking on your blackberry or iPhone, if you have one, can take too long.
Even more important is to keep a pad and pen next to your bed, just in case you have an idea in your sleep and you wake up at night. Many many authors and writers have done this.
A funny story about Keith Richards: he came up with the guitar riff for the song "Satisfaction" during the night. He woke up, turned on a tape recorder, played the riff, and then fell back a sleep but left the recorder on so you could hear him snoring.