It's impossible for your helmet's visor to remain rain-free when you're on the road in a storm. But routine maintenance and attending to the visor as you travel will greatly improve your visibility during wet weather.
Products like Rain-X Original Glass Treatment are designed to repel water glass surfaces. Wipe your visor with it before you go riding, and store some in a small bottle in case you need to clean the shield of ice, debris or bugs while on the road.
Motorcycle apparel manufacturers sell mini rubber squeegee blades that you can attach to the fingers of your riding gloves to wipe the face shield. Carry several of them with you while you're riding in case one of the blades slips off your finger.
Car owners use a soft leather chamois cloth (also known as a chamois) to wipe water off the vehicle after it rains to keep the water from forming spots. Keep a chamois in your gear box to clean your visor in the rain.
If you haven't waterproofed your visor and you're caught in a storm without any of the aforementioned tools, wipe the visor with your riding gloves. Also, if you turn your head to the side for a few seconds, the wind will blow the rain off the visor.