Are you literally just standing around?
My biggest tips: comfortable shoes! Wearing my Danskos, compared to my sneakers, makes a world of difference. Don't be too proud to have a seat somewhere, or lean up against something. Everyone gets sore. I've noticed that whenever I get pain across my upper back and scaps, it's usually because of how exactly I'm standing. Generally speaking, it's because I've been crossing my arms all day and creating a lot of stress across that region. When I notice it happening, I make a concerted effort to cross my arms behind my back instead, or keep my hands in my pockets.