Follow the manufacturer's care instructions for your helmet. Use only the mildest soap recommended. Avoid any petroleum-based cleaning fluids, especially if you own a polycarbonate helmet. Exposure to strong cleaning agents can cause the helmet to decompose and lose protective value.
Keep your motorcycle helmet in a helmet bag when not in use. A simple barrier between the helmet and the outside world can protect it from harm. Without a helmet bag, your helmet is subject to chemicals, fumes, scratches and bugs.
Keep your helmet's face shield clean. Normally, mild soap and water with a soft cloth will do the job. If it gets scratched, replace it. A scratched face shield can be difficult to see through. At night, it could dangerously distort your vision and your view of oncoming lights.
A helmet looks tough and sturdy, but it should be handled as a fragile item. This means that you don't want to drop your helmet onto hard surfaces. It could ruin your helmet. Remember that its function is to absorb impacts.
It is not wise to store helmets near gasoline, cleaning fluids, exhaust fumes, or excessive heat. These factors can result in the degradation of helmet materials, and often the damage goes unnoticed by the wearer. Read the information that comes with the helmet so you know how to care for it.
Definitely read the instructions about painting, decorating, pinstriping, or applying decals to your helmet. Never hang your helmet on the motorcycle's mirrors, turn signals, or backrest. The inner liner can easily be damaged from such handling. In fact, avoid carrying a spare helmet on your motorcycle, unless it's well protected or on your passenger's head. Even the bumps and jarring from normal riding can damage a spare. If it is strapped near hot engine parts or exhaust pipes, the inner liner may distort or melt at the hot spot. The outer shell may not show the damage, but if you've seen the effects of a foam drink cup placed too near excessive heat, you can understand what happens.
When you take your helmet off, find a flat, secure place for it. You could set it on the ground, secure it on a rack, or stow it on a shelf. On some bikes, putting it on the fuel tank may expose it to fumes. If you place it on the seat, make sure it won't fall off. If you plan to use a CB radio when you ride, find a model that doesn't require drilling speaker holes in the outer shell. Before you purchase your speakers, check with your state's laws regulating their use in helmets. Some states prohibit them.
The over all care that is given to your helmet will definitely increase the time you will be able to use it. Your helmet was not designed to last a life time. If you are unsure if it is time to replace your helmet, I would recommend read the article: How often should I replace my Motorcycle Helmet?