I also get significant back pain when standing in one place. I have been using the Hodgman Wadelite belt which helps a lot. It increases the time I can stand without pain from about 20 minutes to several hours. (Of course, if I am standing several hours in one spot, I should question my fishing strategy!)

I also found that keeping my body warm enough is important. Cold water chills the body and tightens up the muscles. Neoprene waders with heavy wool or alpaca socks works for me during the fall, winter and spring seasons. I use light weight breathables only during the warmest summer months.