A recent report by People's Linguistics Survey of India shows that the country has lost nearly 250 languages in the last 50 years and presently people in India speak 780 different languages. (DNA, 18/07/2013)
The best way is to encourage younger generations to speak the language as they grow up, so they will after that teach their children the language as well.
Extinct languages are languages that are no longer spoken by anyone. They can only be revived if there is sufficient documentation of their grammar and vocabulary. For most of the worlds extinct languages there is little or no documentation and they will consequently never be revived. Even when there is sufficient documentation to do this the process is extremely difficult because it requires the presence of a group of people who are dedicated enough to want to learn the language, to use it and to teach it to others. That is very rare. There are only a few documented cases of dead languages without any native speakers being revived. The best known case of such a revival is Hebrew which was never fully extinct because it always had second language speakers.