Ever see a parking lot treated like a trash can? Make a citizen's arrest....OK, maybe that's a bit over the top, but set a good example and police the parking lot. Put trash in it's place and recycle what you can. You may get some strange looks, but you'll also be providing a visible action that will encourage repeating. Ah, the power of bio-mimicry. Washing your hands when you're done isn't a bad idea either.