Football visors aren't part of any league's mandatory uniform. A player can choose to wear them for eye protection, or he can put one on just to look cool. Regardless of the reason, the visors themselves can and do fog up. Since they're made from the same basic material as hockey visors or diving goggles, the anti-fogging options for those devices can also apply to a football visor.
Apply a commercial anti-fogging product designed for visors or goggles. Products come in spray or moist towelettes. Apply a small amount of spray directly to the visor and wipe it across the surface. For moist towelettes, simply wipe across the visor surface. Allow either to air dry before games or practices.
Pour a small amount of shampoo on the visor. Use a soft, non-abrasive cloth to gently spread the shampoo. Remove any excess but leave the soap film.
Use a specially designed mouth guard that limits the amount of warm, moist air being released into the helmet.