The biographer of Robert Moses, Robert A. Caro, refers to the bridges and underpasses of the famed New York State parkways. These bridges and underpasses are quite low, intentionally specified by Moses to allow only private cars to pass. All those who traveled by bus because they were poor or black or both were barred from the use and enjoyment of the parkland and its "public amenities" by the technical design of the bridges. Even at the time of Robert Moses, a political statement of the form "We don't want them blacks in our parks" would have been unacceptable in New York State. But a technological expression of the same prejudice appeared to be all right. Of course, to the public the intent of the design became evident only after it was executed, and then the bridges were there.
In all cases, a commonplace is a method of compiling knowledge for later use. In digital or analog form, this continued growth of stored ideas and projects is a key driver of intellectual development. Any time you decide to work on a project, you should attempt to collect and categorise all information that is relevant and useful.
Methodically noting and filing resources is a sign of a mature and deliberate craftsman—it is an investment into future learning and projects. Before long, you will begin to reach the point where this collection generates projects and ideas with minimal effort; previously isolated ideas are consolidated and curiousity spurred on.
Effective writing stems from intelligently connecting the dots between the concepts you understand and can articulate. It stands to reason, then, that in order to generate more creativity you must not only add to a knowledge base, but deepen and expand the number of connections within the totality of the network. By establishing and explicitly mapping your knowledge, you allow yourself the freedom to remix information. You will often find that solutions come from previously unsuspected fields or topics—proving to be analogous in some shape or form.
Richard Feynman was fond of giving the following advice on how to be a genius: You have to keep a dozen of your favorite problems constantly present in your mind, although by and large they will lay in a dormant state. Every time you hear or read a new trick or a new result, test it against each of your twelve problems to see whether it helps. Every once in a while there will be a hit, and people will say: ‘How did he do it? He must be a genius!’