I'm going for a thicker tail too right now with my horse Maverick. He's got the length down...it's touching the ground. Now I want some volume to it.
What I'm doing is brushing it, conditioning it (my fancy homemade conditioner made with human conditioner, water and vinegar with a bit of baby oil. You spray it on then leave it in.) adding MTG and then cutting an inch off the bottom (completely straight across) then loosely braiding it. I leave in the braid and take it out about twice a week to brush and condition it again, then add MTG about once a week. After that it immediately gets rebraided and kept in for another 4 days or so. I cut it whenever it starts to get whispy at the bottom.
Slowly but surely, Maverick's tail is becoming thicker and thicker! It now looks good and is easy to braid for shows!