one thing I've found pretty handy to keep the shoulder plates inline is running some strapping from just under the chest collar piece to under the tip of the shoulder pad, it's really easy and works good. I can't remember where but I saw someone asking about shoulder plates and forgot where it was so I thought I'd post here. Personally I prefer Adams method (will probably use on my next shoulderpads) but this was ridiculously easy (yeah its a strip of velcro, I'd recommend buckles or elastic).. Then I just used some more velcro to make a band around my upper arm, it doesn't move at all, would probably work for shin to thigh as well, but my current leg armor sucks so I havn't tried...
Btw you gave me the biggest "ask a ninja" flashback when you put the hood thing on haha.