Mark Twain kept 40-50 pocket notebooks over four decades of his life. He often began one before embarking on a trip. He filled the notebooks with observations of people he met, thoughts on religion and politics, drawings and sketches of what he saw on his travels, potential plots for books, and even ideas for inventions (he filed 3 patents during his lifetime). Many of his entries consist of the short, witty, pithy sentences he is famous for. He felt that if he did not write such things down as they came to his mind he would quickly forget them.
Gave this advice: "Always carry a notebook. Write everything down. When you have an idea, write it down. When you meet someone new, write down everything you know about them. That way you will know how much time they are worth. When you hear something interesting, write it down. Writing it down will make you act upon it. If you don’t write it down you will forget it. THAT is a million dollar lesson they don’t teach you in business school!"