I believe I have a problem with internal motivation, in the sense that I have none, or very little at best. This doesn't lead me to, what one might call, "procrastination". Rather, it just makes me idle; content in doing nothing. I say "content", but obviously I wouldn't be asking this question if I didn't see it as an issue!
However, I don't just sit around doing nothing, a lot of the time. I attribute this to external motivators. This applies, as far as I can see, to almost every aspect of my life. To give an example: I know how to cook, but if I'm cooking for myself, I'll probably be lazy -- I might just eat something straight out of the packet! -- but if I'm cooking for myself and others, I'll go to the trouble of making something good.
I've recognised this in me for some time; the reason I bring it up now is because it's related to procrastination -- which was a hot topic on HN a couple of days ago -- and because I'm interested in doing a PhD. That requires enormous amounts of internal motivation, for several years straight, when there's little-to-no external motivation. I see that this could therefore be my downfall.
How does one develop and maintain internal motivation? What can one do, for example, to renew their enthusiasm if/when it starts to dwindle? Besides "passion" and "enthusiasm", what are other people's internal motivators?
The big aha moment for me was in the separation of systems and goals. I always struggled with the same kind of procrastination as you--if there was a pressing need, I'd rise to the occasion, but otherwise was content with the minimum--despite having large ambitions (am also strongly considering a PhD, weirdly enough). The book does a nice job of explaining goals as things that are distant, and so can really never be "pressing" or rewarding short term, whereas systems can have a pulling effect i.e. I don't have one singular side project with a schedule that I'm constantly failing to hit, I have a system in which I write code for fun for at least an hour each morning, during which time I let myself listen to new music and try random ideas while I have coffee.
I might not hit peak productivity, but I do it everyday because I enjoy it. I look forward to it when I wake up. The system pulls me in, and as a result, I'm actually way more productive overall.