I've been on four or five dives so far and have some suggestions:
1) Bring duct tape to seal up bags that have been torn by us (it seems to happen a lot) or by others who were there before.
2) To avoid confusion, bring stick-on labels to place on bags that have already been explored.
3) Clean up not only after ourselves but pick up anything that was strewn about BEFORE we got there and put it in one of the bags. Take "before and after" pics, post them online, and leave a note for store owners telling them how to access the photos. Ask if they might consider dealing with us directly so we won't need to go digging through the trash.
4) On planned dives, print up a list of the places (with addresses) that are going to be visited. Or announce this list before setting out so people can jot it down and avoid losing the main group.