Regular checkups will allow your dentist to detect any problems such as periodontal (gum) disease, a dry mouth or other disorders that may be the cause. Maintaining good oral hygiene, eliminating periodontal (gum) disease and scheduling regular professional cleanings are essential to reducing bad breath.

Regardless of what may be the cause, good oral hygiene

 is essential. Brush twice a day and clean between your teeth daily with floss or interdental cleaners. Brush your tongue, too. If you wear dentures, be sure to remove them at night and clean them thoroughly before replacing them the next morning.

Source: http://www.ada.org/public/topics/bad_breath_faq.asp