Something I can't emphasise enough through having worked as a DMP for a few years now is having a comprehensive understanding of traditional art principles. These include drawing, light and colour, perspective and composition. These are the foundations for every DMP I've ever worked on and before landing a job in DMP in my opinion one should have a firm grasp over them.
From studying Animation Production for three years one if not the only things I gained from doing so was being able to further my drawing skills. By no means am I somebody with great skill but am able to draw technically sound from observation. What learning to draw will teach you and is vital in my experience when it comes to DMP is scale/ proportion through having to observe and measure along with such other things as perspective. One of the biggest things you will learn from drawing is being able to record what you are seeing. Being able to observe and understand what you're seeing and replicate it comes in very useful when working in DMP.
In its most basic form an example of this could be modelling a house. Being able to get all of the proportions of the house accurate is very important. Another example could be if you look at a plate but don't understand where the points of perspective are or take them into consideration then you're going to have issues.
Something I soon realised after I finished my university degree was that although I could draw my understanding of light was somewhat lacking. In order for me to improve my understanding of this I read up on the subject as well as watched video tutorials.
What immediately jumps to mind and for me and should be read by anybody who needs to improve their understanding of light is Light for Visual Artists by Richard Yot along with watching the gnomon workshop DVD Practical Light and Colour by Jeremy Vickery. In relation to perspective and the fundamentals of composition a great book to read is The Art of Layout and Storyboarding by Mark Byrne.
Something I do on a regular basis and for me is a key part of learning traditional art fundamentals is always observing the world around. Continually I find myself asking.. Where's the horizon line? What points of perspective am I seeing? Where is the direct light coming from? By observing the real world and understanding what you're seeing you're rehearsing the typical questions that arise when dealing with a DMP.
A little after my graduation I had the pleasure of speaking with a senior DMP who gave me the essentials of what I needed to do to become a DMP. The first thing he mentioned to me and what I've just mentioned to you is that a comprehensive understanding of light is absolutely key in DMP. My realisation of a necessity to further my understanding of light came through having spoken to this person and as soon as I did so things started to make much more sense in terms of what I was doing.