The following is the way I've approached the problem, and it seems to have worked for me. I've never tried to see if it would work with somebody else before, indeed I don't think I've ever explained this to anybody else before.

As I see it, these problems arise when what I think I should do, and what I feel like doing are in conflict with each other. Going with what you feel is easy, it's sort of like the automatic mode of operation. Overriding this and acting on what you think takes effort, and the stronger your feelings are wanting to do something else the harder it is.

The trick then, is to try to reconcile the two. The way most people do it is that they starting doing what they feel, and then rationalise it to the point that it's also what they think to some degree. Fortunately, you can also do it the other way as your feelings are trainable. Find what ever it is that you want to rationally do, and then keep on reminding yourself not just why you want to do this, but also try to feel it. Imagine how doing well in, say, some course of study is going to benefit and advance you in the future. How it will give you an edge against others who haven't studied the harder aspects of it, etc. Be creative, think of all sorts of positive reasons why doing this thing that you already know you should do is a great thing for you. And, most importantly, try to feel how you will benefit from this. Imagine yourself in the future having kicked butt in this course, or what ever, and imagine what that is going to feel like. Really try to feel it!

It takes time, but you slowly build up positive emotions around these things that you should be doing. At first, it just doesn't take quite as much effort to do them. Then it comes quite naturally. And after a while you will find yourself actually wanting to do it, to the extent that it would take an act of will power to not do it. Really.

This process itself also becomes a habit. When you decide to do something, you will automatically start to build up positive emotions around what ever it is that you've decided to do. During my PhD writing it built up to such a degree that I'd have these dreams some nights about how amazingly happy and proud I was going to be when it was finished. Motivating myself to work on it wasn't a problem.