Here's the list (I know, it sounds like nagging, but do these things anyway): wear your seat belt, drive defensively, avoid situations that may lead to injury, avoid risky sex, avoid violent situations, don't smoke (or quit smoking if you do smoke) and maintain a healthy weight.
If you can do those things, then you are already increasing your life expectancy. Focus on avoiding obvious risks and dangers. Your body is pretty amazing and will keep going for a long time as long as you keep it out of trouble.
Read More: Major Causes of Death in Young Adults