When I´m not at home or in a place where I can draw comfortably, I carry around a small notebook where I can sketch very quickly any idea that I have in my mind. Of course after time passes not any idea I have is still that nice and I abandon them many times, but it is a good thing to carry around. Here are some pages. In this sketches the quality of the sketch is not important at all. What is important for me is not to lose the idea. Also I use what ever pen or pencil I have with me at that moment.