First and foremost, I believe the most important factor is good nutrition. Without the nutrition, your horse will lack the muscle development needed.
Here is a great article that I find helpful:
Secondly, another important factor in the equation of developing a strong topline, is saddle fit. If your saddle does not fit correctly, that will only hinder or impede muscle development, and only create the reverse effect.
Muscle atrophy is very common in horses who are going around with an ill fitted saddle. If the saddle isn't allowing the horse to relax uptop, to stretch and lift - then you wont get far with developing a topline.
Here is a great discussion on proper saddle fit, with wonderful video's put out by Schleese to help you decipher if your saddle does, or does not fit. *Remember, this does not replace having a Professional Saddle Maker/Fitter assess your horse in person, 1 on 1*
Once you have figured out a good nutirional balanced diet for your horse, that will help develop muscle, and know, that your saddle is a correct fit for both you, and your horse, the next step is to help develop the topline from the ground.
Here is a great video that does a great job showing the steps and stretches that you can do with your horse on a daily basis to help loosen, relax and work the back.
And of course, under saddle work. Lots of great advice given here on HF, on exercises to do, to help your horse develop that topline.
Long and Low Work
Raised Trot Poles
But you have to know, that in order for your horse to be effective during these exercises, you, the rider must be as well. You must know how to ask your horse to open up front, track up, and lift their backs up into your seat. You must be effective, in order for your horse to be as well.
Feel free to add your experiences and your input on the matter. The more the better!