My first concern is always that the shampoo and conditioner do not contain sulfate or other chemicals because they strip the skin and coat too severely and can cause dryness and itching.
There are several all natural brands out there now. Isle of Dog is one. I believe BioGroom is also all natural. I usually use the YSS products. They smell great and leave her coat shinny. Here is a link:
http://www.yorkieshampoo.com/
I have also used WEN on her. My groomer uses WEN and Gracie always looks and smells so wonderful when she comes home. They are both great products.