The most common cause of bad breath is bacteria on the back of the tongue and between the teeth. Both flossing and tongue brushing may be required to remove bad breath.
While bacteria between the teeth is a commonly-recognized cause of bad breath, if flossing does not resolve the condition then tongue cleaning/brushing should be tried in addition.
Tongue brushing involves scrubbing the tongue with a toothbrush. Tongue cleaning utilizes a U-shaped scraper which is drawn over the tongue from back to front. Tongue cleaning causes significantly less gagging than tongue brushing and is a common practice in some cultures. To greatly reduce bad breath, brush, floss and scrape at least daily.