The Danish design agency Saatchi & Saatchi has made this pretty simple and straightforward environment ad:

To make people realize that saving the planet starts with them saving paper, we took a standard paper dispenser and made a simple modification with green foil and the silhouette of South America. This allowed us to prove that the survival of the forest is directly connected to what people consume.

You can see a larger version of the ad here. You can also check out this clever ad on a bridge in Amsterdam.