A helmet is essential for any kind of two-wheeled vehicle, but some people resist wearing one because of "helmet hair". I can't help thinking that it's more important to have a functioning skull than good-looking hair, but if it's an issue for you, the Airhead might be the solution.
The Airhead is a plastic insert for helmets that improves air circulation, reducing perspiration and (hopefully) stopping the worst excesses of helmet hair. At $25, it seems like a pretty sensible investment. If you've given it a try (or have another solution), tell us about it in the comments.