What steps can be taken to reduce massive tooth decay?

Ideal oral hygiene! Floss daily (by far the most important thing you can do, ); brush carefully, watch your intake and frequency of sugars. An unacceptable alternative would be full mouth extractions. No teeth ..... No decay!
Diet and oral hygien. Cleaning you teeth with good brushing and flossing is essential. You need to have a diet low in sugar. Drink water and avoid soda, energy drinks. Flavored ice teas, even diet drinks are bad for teeth. Using xylitol mints or gum after eating will help to eliminate the decay causing bacteria. And have your decayed teeth treated!
See a dentist. Get the decay treated, get oral hygiene instructions and a cleaning. See your dentist on a regular basis. You are fighting a losing battle without professional care.
Prevention and DDS. You can prevent tooth decay with good oral hygiene- tooth brushing, flossing and a diet that does not promote tooth decay. Once a tooth has decay, it needs to be treated by a dentist. Otherwise it will just progress and lead to tooth loss.