I scrape and brush my tongue to remove the white film. I clean my teeth. I drink herbal teas such as peppermint teas. I do not chew gum on a fast... as a matter of fact, I chew nothing on a fast. Chewing can trigger the digestive system, making you feel very hungry.
Just keep your mouth clean like normal, rinse with water often... drink minty herbal teas...
I did not have an issue with bad breath on any of my fasts.