As awful as this sounds, I think it might be easier to train crows to pick up people's trash than to train people how to throw away their trash. People are apparently not as easily teachable as literal birdbrains.
And I am sure that if you were to offer people a dollar as a reward each time they bring back a trash to a trashcan, the parks would be clean.
In Berlin glass bottles are reused and there is a reward for bringing them back to specific bins. I have seen homeless people looking for trash and filling carts of these.
It is not only homeless, people with very low pensions or not able to get a proper full time job also hunt for them, everywhere.
On the streets, trains, subway, beach, lakes, you name it.
To the point that many people don't throw away bottles anymore, leaving them grouped outside the trash cans, as means to help those people pick them without messing with trash.
There are even trash cans in the Rhein region that have bottle holders around them.
So yeah it helps keep the street clean from bottles, but fighting the causes of this 2nd level economy would probably be a better idea.