If you horse has thin, soft soles, you're probably dealing with your mount's constant "ouchiness" when you ride. Hardening the soles of a horse's hooves may require maintenance changes along with application of topical hoof hardeners. Regular use of these hardeners can help prevent sole bruising, or at least make bruising less likely.
Your horse's feet become softer when exposed to a frequent wet/dry cycle. This forces hooves to constantly expand and contract, weakening them in the process. That happen if your horse stays in a dry stall at night, then is turned out on wet grass in the morning. In addition, your horse's soles are exposed to ammonia in the stall from urine, which also weakens them.
There's no shortage of products on the market aimed at hardening soles. Ask your farrier for a recommendation -- he should know what your horse needs and what works.
Pulling your horse's shoes and allowing him to go barefoot can toughen his soles, but it can take a long time. You can still ride your horse during the toughening phase by using hoof boots, which support the soles. Hoof boots aren't one size fits all -- you'll have to measure your horse's feet and find the correct size for him, which can vary by manufacturer. For best results, trace a copy of your horse's foot on a piece of paper and bring it to a tack store to compare sizes.
Because of genetics or hoof conditions such as laminitis, some horses will always have exceptionally thin soles. Putting pads on their feet when they are shod can protect the sole and allow the animal to be ridden or driven.