My dog Chico had demodex pretty badly. We gave him Missing Link supplements as well as fish oil. It's really important to give the fish oil, you'll start to notice a diference pretty quickly. Good luck, and be patient!
here is where I buy my Missing Link, I think the price is pretty good on this site. Only buy the small bag though because it will spoil if not used quickly enough. http://www.dog.com/itemdy00.asp?T1=221130+108&Cat=