I’m reminded of the question, “How do you get to Carnegie Hall? Practice!” 
Almost everything I learned was On-the-job-training. I got interesting animation in the 70’s and started with claymation techniques. I got interested in computers about the same time and wrote a (very simple) program to computer animate shapes.
Pre-viz work is a multi-discipline (at least as I and my team practice it.) You have to have a sense of character, story, and plot more akin to an English major than Art student. Then you need solid art skills in some discipline. I’m an odd-duck since I consider my 2D work only passible (looking at your portfolio, you’d run rings around me) but I lucked out that “sculpters” were needed for maquettes and other other trial ideas when I started. Almost everyone work with has a strong sense of Seqential art (Eiser is a good teacher for this.) It is used to storyboard and to find the character’s “moment” when you’re designing attitudes. A healthy does of comic books as a child seems to actually pay off in this case. 
Skills-wise, a sense of “design” comes into play. When doing previz you have to have a “purpose” for what you’re protraying and usually this needs to be aligned to the producer’s or director’s “vision” of the story they want to tell. Check your ego at the door, because you often have to align your designs in a meaningful and clear way with the director’s or the lead animator’s for whatever you’ve been asked to do. You really have to like working with people and have a desire to produce visuals that please them. Artists which sometimes have too much “artistic temperament” don’t last long in the previz/production design game.
For example, I do a lot of advertising work. Me and my team have sometimes busted ourselves for days to present the client with what we thought was a “brilliant” portrayal of an advertising sketch or campaign. The client hates it and we have to pocket our egos, re-interview the client and come back with something the client loves. (My world obeys “The Golden Rule of Arts and Sciences” which is: “He who has the gold, makes the rules.”)
Looking at your portfolio, I can already see that you have some good base skills as far as your draftsmanship and artistic sense goes. I would concentrate on “Story” and “Sequential Art” to round out your porfolio more. Contact me by PM if there is more I can tell you. 
-K