My TB gelding had a REALLY thin tail. It was horrible and straggly and it didn't look nice plaited at shows AT ALL! I find that many other people have the same problem, so here is what I did to make it thicker;
I untangled the knot with my fingers and brushed out the tail with a body brush.
Since the ends were so uneven and straggly, I cut off about and inch.
Then I washed it with shampoo and conditioned it. Once it was dry I ran a comb through it (pulling tangles out with my fingers) and put it in a tail bag. I pretty much left it bagged all the time and never brushed it. I washed and conditioned it every 2 weeks. Within 6 weeks his tail was fuller, silkier and it much better condition.
If you are keeping it bagged, then I recommend using a little baby oil to soften the hairs. Don't use baby oil if the tail is going to be exposed in the paddock, as it can act like a magnifying glass and burn the hairs.