A trick I often use when going to greenhouses/butterfly sanctuaries is to use a UV filter (extra protection), and pay a visit to the bathrooms there in order to place the camera under one of those hot-air hand-driers. Then a short while later, hurry into the humid/hot area where you will be shooting.

Breathing on a lens I really wouldn't recommend, since breath acids can damage delicate lens coatings. And there is also the possibility of spit landing on them too...