What effective innovations can be explored in sustainability-related policies, regulations, mandates and incentives that are both practical and have the potential to make a big difference?

Every city, state and country in the world is announcing policies related to sustainability and climate change. While some of these are good, many of them are average and some of them, downright mediocre. What are the policies that have done really well, and are worth emulating by others? What could be some other policies that have not been tried much but have significant potential?