Norma, I also have shoulder length hair and my two sisters in law took turns to wash my hair when I was in the hospital during radiation treatment with my neck seriously bruised. Use baby shampoo as suggested in the previous post. I didn't have plastic wrap (maybe a good idea to have it) but I had two towels around my neck and chest to make sure the water didn't get to my neck. It was not that difficult as you imagine even when I had the IV tube on my left arm. I sat on a stool at the washroom bowing my head down the washbasin. One important thing to remember is the action has to be quick and the hair has to be dried thoroughly with a dry towel before leaving the washroom. Use a hair dryer with a low heat afterwards.
I felt so happy when I could wash my own hair a few weeks later!

Karen stage 4 tonsil cancer diagnosed in 9/01.