You're never going to get a realistic hard armor rig that will follow the character in certain areas, not without an impractical amount of work.  Avoid putting any straight, hard-looking structure in places where bending is going on, like at the hip or endpoints of chest or abdomen.  You can fix some of it with JCM but the more complex the model, the harder that is.  It does help to look at how real armor is designed though (that strap right under the armpit is just not how a pauldron is constructed).

You have a lot of leeway with science fiction/fantasy armor, because you don't have to follow traditional designs, but certain things are just not very ergonomic at all.

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