Business for Social Responsibility (BSR) recently released a new report titled Eco-promising: Communicating the Environmental Credentials of your Products and Services. According to BSR, the report will help readers "learn how to reap the benefits of your environmentally friendly practices and avoid consumer backlash for perceived greenwashing."
Just read the report, it has several great examples of how to approach it effectively. I love the Boots example, putting the onus on customers to do their part.