Work and diet will strengthen a hoof. If the frog is weak I would ensure there isn't even a hint of thrush and get your farrier/trimmer to check that the heels aren't too high.
In this dry weather hosing and aqueous cream help keep them moisturised too.
Shod horses often have poor frogs as they are unable to function correctly due to the shoe.The strength of the horn, its thickness and durability are down to genetics. Some change in nutrition can help but it will take several months before you will see a difference. Massaging the coronary band can also be effective as it stimulates the cell growth but again in will take several months before any real change can be seen in the quality of the hoof. If the horse is barefoot then asking him to walk over firmer or even stony ground (not sharp stones, think along the lines of cobbles) will also help.