Looking at this from the perspective of a student, I never wanted to write out all of my working because to me there were steps that seemed obvious, and all that was needed was the answer. The most effective incentive in getting me to show my full working was always the same: an answer alone, if incorrect, gets zero. Not just no marks, but no help either. An incorrect answer with full working shown might get you partial marks for the bits that are correct, and the teacher can point out where you went wrong along the way.
I remember many questions that made this explicit by presenting an initial question, and then saying something like this:
a) Find the value of x
b) Use your answer to a) to prove this other thing.
c) Using your answer to c) (or otherwise) show that y = 2 in this other, related equation.
As you progress, exams stop phrasing it like this and start lumping them together into "Find the value of y in this equation" and expect you to work backwards, using the other thing from b) above in order to get a simpler equation, solving that for x and then substituting back into the first equation to solve for y. Having seen questions like this broken into parts before, students tend to spot that this question is worth a lot of marks and realise that there are likely to be marks for intermediate steps.
One way to help here is to let the students see the mark scheme, or at least an example one, so that they can see the breakdown. When students see that a 15-mark question only gives 2 or 3 marks for the number at the end, and the other 12 or 13 marks are spread around various key stages of the working, they'll start showing the working.
Essentially what this boils down to is this: if you want students to show their working, make it worth doing. People (students especially) are much more likely to do something if there's an actual benefit to them in doing it. This may be a better shot at getting the marks, even when they go wrong partway through, but even when the mark scheme isn't yours to control, you should stress that showing their working also lets you point out where students went wrong so that they can identify and improve on anything they don't understand fully. For me, that was often a stronger incentive than getting the marks.