Try looking for unusual compositions, billboards or signs. Then, frame your composition and wait for the right person or people to walk into your frame to complete your pre-visualized idea.
I liked the design on the wet wall and the placement of the tree in the above image and made several images with different combinations of people and at different shutter speeds so as to experiment with various styles of blur. To be sure, your experiments can keep all elements sharp–I just happened to be experimenting with movement in this case.
Among the experts at this kind of “juxtaposition street photography” (as well as street photography in general) is John Crosley.