Be an example! I'm sure that most people out there don't like to pick up trash. It's yucky, right? Dirty. Don't want to soil your hands, yes? But here's the thing. If one person sees you pick up trash, maybe, just maybe, he'll think about not throwing something on the ground next time.
Also, all that crap that you see in the gutters ... where does it go? Well, if you're lucky, it will get filtered out somewhere, but odds are it will end up in the water system. And then what? Eventually it ends up in you!
So be an example. Pick up garbage when you are walking on the street. I often pick up an errant plastic bag and then fill it with trash. Find a garbage can and then at least the trash has a better chance of ending up in the landfill - not the best solution, but a hell of a lot better than being in our water.
In case you don't watch the video below (it's less than 6 minutes, so watch it!) - here are a few facts about cigarettes: 15 billion are smoked worldwide each day. 40% of the trash that washes up on beaches is cigarette trash. The filter is a plastic derivative that holds toxins in it - when it hits water, those toxins leach out. $4 billion is spent each year to clean up cigarette trash. Yuck!