The foam method is pretty popular, using EVA foam mats to shape armor plating, as they can be cut easily and bent into shapes with a heat gun or hair dryer. Alternatively, you could sculpt the armor out of clay over a manikin and then take a mold of that and cast it out of latex.
Do a little more searching around the forum, there's a wealth of knowledge here on these two methods, as well as pepakura and plastic armor plating (which is what I'm into). Good luck on your build!