Which brought me round to the well-known concept of sense of ownership in communities. When people feel alienated, and feel that their environment is nothing to do with them, they no longer FixBrokenWindows. On the other hand, empower people, make them feel they have a stake in their environment, and they will.
This principle has been behind the regeneration of many concrete tower block monstrosities -- turning urban wasteland (which planners said could only be fixed by bulldozing) into communities. Note the emphasis on CommunitySolution rather than TechnologySolution. A TechnologySolution would be to raze everything and rebuild. A CommunitySolution works with what there is (though it's not ideal -- cf Ward's thoughts on how Wiki's imperfections help foster a community).
This is the core idea behind Wiki, and SoftSecurity: it belongs to everybody, and therefore everybody has a stake in it and an interest in protecting it. You could call it fuzzy self-interest I suppose.
Hence: no "Keep off the grass" signs. The grass belongs to everybody. Keep on the grass.