I did a lot of looking into this stuff and what I found is that lots of people say "Use Shampoo", but here is the catch: doll hair isn't real hair! It doesn't need Vitamin E, Aloe, and all that other stuff that comes in shampoo - and if you use it often times what you get is a residue that starts to gather dirt (yes, experience talking).
Eventually I ran across the following tip that has worked wonders for my old Jenny dolls and such: Dishwashing liquid and fabric softener!
Dishwashing liquid, like Dawn, will clean the fibers of your doll's hair/wig and not leave residue on it. If the wig/hair is extremely tangled and messy, soak it in fabric softener or woolite for a while and it will help detangle and act like a conditioner. DON'T use dollar store fabric softener - it can be nasty and leave residue as well as a disgusting smell (yes, once again this is experience speaking). For the sake of your doll, buy something good!
For static control gently rub a dryer sheet on the hair/wig. When I'm rooting a 1/6 scale head I actually wrap my rooting hair in a dryer sheet so it doesn't build up static as i work. Mind you I live in a very dry climate - those in a more humid area may not have that problem LOL