> Walk around with a pen and a scrap of paper.

From http://www.diyplanner.com/docs/commonplace1:

"In Sir Arthur Conon Doyle's tales, Sherlock Holmes kept a large and sprawling commonplace book upon his shelves, essentially his own home-grown encyclopedia of people, places, crimes, and pertinent facts, like the colour of mud from various locations, or the consistency of tobacco ashes. In the middle of a case, Holmes would rush to his commonplace, haul down a book, and read out a newspaper article concerning (say) the disappearance of a hydraulics engineer a year prior. His attic of a brain remembered vague references, but the books held the details that allowed him to pursue his work."