Personally I wash out my horse's tail aprox every 1 - 2 weeks now.
Before washing I unbraid and brush gently but thoroughly.
I use a dandruff fighting shampoo medicated with vitamin e and fish oil.
I make sure I massage all of the goodness in and make sure it has a good lather, then rinse. Afterwards I let it air dry to about 90-95%.
Once it is mostly dried, I take my olive oil/heat protectant homemade mix and drop about a quarter size into my hands, rub them together then gently massage the product into the tail around the tailbone. Then I use the exess and spread the rest all thru out the tail. Often times I'll use just a little bit more for the tips to keep them moisturized. Then I brush thoroughly again, making sure that no hairs are pulled, then I braid and BAM! Done for 1 week. Then I brush and repeat braid and/or rewash :) His tail is so soft it's like silk!