#1. Become a baptized, dedicated member of the Jehovah's Witness organization. Use every opportunity available to convince other people to join. Attend local meetings and follow the organization's rules and reccomendations in your daily life. Make sure you are in "good standing" when Armageddon arrives to destroy wicked people.
#2. Wait for human cloning and brain surgery to be perfected by medical science. Then do the following steps:
A. Have yourself cloned.
B. When you reach the age you no longer find acceptable, have your brain transplanted into the younger version of yourself. Continue to live your life in the same body you had when you were younger* until you once again reach the age you don't want to go beyond.
C. Repeat the above steps indefinitely.
*The life of your clone is another story.
#3. Make your mark on the world. Teach your children, making sure that the quality of life for the next generation is better than what you had, teaching your children to do the same for their children. Make a difference in the world and do something to further the advancement of the human race, even if in a small way. Make your name remembered.
Worry less about how long you've lived and more about how you lived and who you were.