Back in the 90s there was a Japanese TV documentary on street cleaning. A mobile hot dog stand served as a test. During two weeks the staff did not clean the street in front of the stand. Leaves, some trash, the usual detritus accumulated. During weeks 3 and 4 the staff would...

...go out to clean the street carefully every morning before opening. Sales picked up noticeably. People were more willing to shop from a hot dog stand in a place that looked clean/welcoming (shoppers and people who just passed without shopping interviewed). During weeks 5 & 6...

...they would clean the street in the morning and again during rush hour, or whenever they did not have customers. Sales shot up dramatically. Shoppers explained that they were impressed with the dedication of the staff. Lesson learned: clean your street and be seen doing it.

No matter how virtuous the population, litter happens: people drop it, wild animals or wind spreads it around. Keeping your city clean is hard work and fostering the mentality that recognizes this is valuable. In Japan there are even books on this subject, Soji-do. 掃除道.