You are going to want to invest in hoof supplements and a conditioner. A horse's outside appearance is directly related to their nutrition. You have to start from the inside and supplement that process with topical treatments. Hoof polishes are purely cosmetic and can dry out your horse's hooves, thus why they are used sparingly only at shows.
A horse's hoof is dependent on natural moisture and weather conditions. In extremely dry climates it will be important to add oils to help keep the horse's hoof from drying up and becoming brittle. When applying oils to a horses hoof, it is often recommended to only apply it to the area just below the coronet band. This will allow any natural moistures to be absorbed as well.
The opposite end of the spectrum is a very moist, wet environment. The horse's hoof structure when living in damp environments will weaken. There are hoof conditioners that are used to harden a horse's hoof in times of extra dampness. Thrush will be found more often in this environment due to the increased bacterial breeding moist environment creates. It is common to use a thrush treatment formula when hardening up a horses hooves because it treats the extra moisture and treats the infected area. When applying a hardener, it is common practice to apply it directly to the bottom of the hoof.
I have also never heard about the watching for nail holes thing. Complete bogus.