No experience here... I'm still a newbie, however I've been on the flip side of the coin most of my life. I would say it all originates with the quality of people that rent in your complex. Professionals who take pride in how they live, will probably take better care of a property than someone of a different social class. Hate to sound like a snob, but it's a reality. Growing up in NYC, the buildings I lived in housed some pretty interesting characters. I also got to see some of the ritzier apartment complexes. Huge difference!
One interesting method of screening a tenant (which I learned on one of BP's podcast), is to look at how they keep their car. Somehow, find a creative way to take a peak! lol! sounds funny, but can be revealing. If they are slobs with how they maintain their car, they will be slobs in and around your complex.
I would say that renting to older retired people may be one another option to look into. My REI niche is mobile home investing. I have seen the way older people live in the 55 and over communities, versus family parks. Without question, they seem to take great pride in how they live and how they keep their grounds. They also tend to watch out for each other, and report things they see.
Another thought that comes to mind is getting involved in a neighborhood watch program. This could also add value to your property, as it may help bring people together and keep out the riff raff.
Putting up signs and creating newsletters/mailers addressing the littering may be helpful, as it shines a spotlight on the problem at hand.
And finally, the most extreme and drastic approach... Putting up cameras around the complex (or fake ones)... just a thought! :)