I find that human beings are motivated more by social benefits than rational self-interest. If it is possible given your age/gender/current size, focus on getting a sexy body and on how cool and attractive it will make you to the people around you. If you can successfully obtain that goal, which is slightly adjacent to the goal of being healthy, you will already have experience maintaining the habits of exercise and watching what you eat, and you'll be well equipped to continue to do as such for as long as you desire.

This has worked for me pretty well for six months. I'm addicted to working out.