I find it hard to make curving lines when major areas are covered with armor, which is otherwise rigid. :S
Good point. Perhaps they are not organic on their own, but when we apply the separate rigid pieces of armour onto an organic body, they begin to follow the natural form of the figure underneath when we look at how they are arranged relative to each other.
On the left we have a male with a suit of armour on. His (very rough) line of action implies a sense of balance and weight; his torso and legs are leaning backwards slightly (not very visible from this angle) while his pelvis is thrust forward. The humanless suit on the right, however has a much less natural line of action and appears to balance its weight with much more rigidity.
Here's a more practical example:
I'll admit there is a bit of cheating going on here, seeing as the torso and helmet pieces probably don't allow for that much flexibility, but the principle still remains.
Armour isn't exactly completely straight in shape, either, it just cannot bend or distort itself. It still retains some of the form's natural elements, albeit in a much more angular fashion.
The best way to get a firmer grasp on drawing figures naturally is just by drawing them from life and concentrating on retaining the information you see. If you make it a point to study people in armour and such you'll find that it eventually just comes to you instictively.
