One thing we did a couple summers ago is to have a big block party in our alley to bring the neighbors together. Before the block party, we had an alley clean up day. We picked up trash, put it in the trash cans, and got it out of the way for the party. Then we had a lot of fun grilling burgers, drinking beer, and chatting until the wee hours of hte night. I think once people realized how usable the alley could be and started to get to know their neighbors, people started to think of the alley as shared space and most are better about keeping it clean. We still have some problems with cars parked out into the alley, but by and large folks are better about it.