Hi Jubilee Rose,
I have an ex racer, and we she first arrived she had no top line to speak of! She particularly lacked any muscle around her 'butt'!
I found a combination of the following has developed her... she's short-coupled and looks a complete power-house these days!
1. Fed her extra (in line with everyone else's advice!). Important with most thoroughbreds though, is not to feed them with a mix too 'heated'. They tend not to need any more buzz!
2. Lots and lots and lots of trotting work. This also helps with balance and straight movement.
3. Ensuring that she is riding correctly to the bit - I think this is the most important part. Lucy didn't correctly use her hind legs... going for the easier option of hollowing her back and trailing them. Rather than trying to get her to accept contract straight away, I taught her firstly to balance herself and work straight then how to stretch down to the bit (nose down to her knees, almost), which stretches her top line and helps her to step under.
This exercises the haunches and helps develop muscle tone.
4. Using massage after exercise. Although this doesn't strictly build up muscle, it helps tired muscles to recover and be ready for the next training session. It also helps to check for any knots in the muscle or muscle development that's 'one-sided'.