Trot-work really helps. This worked a lot with my mare. This summer I barely cantered her, I just worked a lot on trot and it really improved her topline. The thing you want most out of your horse's trot though, is collection, which is created by impulsion. When your horse is working through its hind end, and creating impulsion from behind, they are able to better collect themselves and be "on the bit." When a horse tucks its back end underneath himself, gives the rider his head, he is offering his back to the rider. You will feel the difference, especially at the sitting trot. If the horse has its butt and nose in the air, its back becomes hollow and it isn't forming very good muscling through its back and neck. Work at getting your horse supple and on the bit at the sitting trot, and you will feel him "offering" or rounding his back to you. This will really help with the topline because the more your horse works on the bit, the more collected, and the more rounded his back will be. It also creates great butt and neck muscle. Good luck!