If the individual hasn't got any intrinsic motivation ("likes the topic", "wants to learn") then you must set up some form of extrinsic motivation ("money", "reward"). You can reward yourself, too: "When I have succesfully learned this then I'll reward myself with something." For some people, the achievement itself is more than enough as a reward.
However, extrinsic motivation becomes increasingly hard, it depends on the needs of the individual. The best method to stay intrinsically motivated is to find groups of other people who want to learn the same thing. Competition and social factors ("Everyone's doing it", "I need to adapt to this culture", "I'll be the coolest guy around with that piece of knowledge!") have a huge influence.
But the most important thing is not motivation. It's discipline, for the sake of it. Train your discipline.