As I was taught on some psychology or pedagogy course during one of my studies, the best one is so called “internal motivation”. But how to find it?
Ask oneself a question – “Why do I want to do this?” And then just remember the answer.
It is easier to say than to do 😛
When I was starting my PhD studies I wanted to do research useful for society. Then I faced reality and I realised that not all researchers do useful research, and even, that part of them don’t care about society at all, they care only to publish enough to receive a salary (maybe it is a problem of Polish science financing system, and parameterisation). Researchers who believe that their job is “for something” are in minority. Fortunately I was lucky to meet some of them 🙂
And it was like a general motivation to start a research/ academic career. When I think about it I have also this post “How To Feel Fulfilled: Tools From Aristotle (or Ari as I call him)” of Karen Salmansohn in my mind.