It's not ideal to use space heaters, because they use energy and add an expense. However, in the last decade space heaters have become safer and more energy efficient. They now come in several forms, such as flat panels (pictured), heated foot rests, heated rubber mats and heated shoe inserts.
The author discusses better and worse ways to influence guests to make greener choices. Some of his advice: write messages positively--don't appeal to customer's sense of guilt; tell them the benefits to themselves and the planet; communicate the sense that ones' peers are already practicing the desirable behaviors; reassure customers you care for their wellbeing and comfort; and make sustainability implicit.