Face it, your helmet (or brain bucket) is one of your most important pieces of equipment. And if you're smart and own a good one, chances are pretty good that you plopped down some serious cash for it. It only makes sense to taking care of it and making it last as long as possible. In fact, there are several things you can do to take care of your helmet.
With a new helmet, the first thing to take care of is preventative maintenance. Use a high quality wax on the outside of the shell. Not only does this signal to others that you've got a good sense of style, but it will make it much easier to clean. As a bonus, it's more aerodynamic.
For periodic cleaning, some manufacturers recommend first removing the pads and liner and do a visual on it. This is a great opportunity to look for damage, especially if you've recently taken a spill. Then use dish soap and water to clean it well. After drying it, try a product such as Honda's Spray Cleaner and Polish. It contains a wax element that will help keep it clean.
Since you've already got the liner out, go ahead and wash it by hand. The best thing to use is hair shampoo and cold water, not hot because some of the glues used can lose their grip with heat. Then, rinse it well and set it out to air dry. Never put it in a clothes dryer.
Finally, one topic that too many riders don't pay enough attention is storage. There are a few things to consider when your helmet won't be used for an extended period of time. First, spray any rubber parts with silicone to keep them pliable.
You'll want to keep it out of the sun. The UV rays will damage the shell and render the color drab and faded. The foam that forms the protective barrier between your head and potentially damaging hard, physical objects is called expanded polystyrene (or EPS). The sun will prematurely dry it out and make it fragile. Too much heat will melt it, so avoid leaving it in the trunk of your car. Any temperature around one hundred degrees makes for a bad environment for your helmet.
The EPS is also sensitive to petroleum products and other damaging chemicals. Keep this in mind anytime you are fuelling or servicing your ride.
Bottom line: take care of your helmet and it'll take care of you.