A lot of great suggestions have been posted already. I just want to add: please remember to rinse out your horse's tail very thoroughly after shampooing. Shampoo residue can dry out the mane and tail even more. You can use M-T-G (search at equine online store) to treat dandruff, if persistent. Also used with good results: go to your local drug store (Walgreens or such) and look for hair products created for Afro-Americans. There are some good natural conditioning products containing olive oil etc. you can choose from. Give it a try!
Other grooming tips can be found here: Horse Grooming 101