Pick up litter.
If you see trash lying around don't just stare at it and walk pass it. Try to make time once a month to go around your neighborhood and pick up all of the trash that you see. Just be sure to use a grabbing stick or gardening gloves, to prevent hurting yourself on something dangerous.
You can also join city-wide groups that go around the city and pick up litter in the most affected areas. Contact your city hall to locate a group for your city.