There are several parts of a cameraman's helmet, many of them far beyond the basics of having something to protect your skull and support a few cameras. I'll try to discuss each of them and list my thoughts on the best type of equipment to utilize. Keep in mind that new technology, materials, and hardware will surely render my ideas as outdated as time passes

Helmets - This is the framework of a camera helmet and should be carefully selected. Full-face helmets seem to be the most popular with more experienced cameramen because they protect your chin and also provide another area of your face (your chin) that can support a heavy helmet. Chins are often very sharp at the edge in comparison to your round head, therefore a full-face helmet utilizes your chin to prevent unwanted helmet tilting or panning around your head. The drawback to full-face helmets is sometimes the limited view, however most newer helmets don't have any problems with visibility. Make sure you purchase the lightest helmet you can afford. That often means carbon fiber or kevlar and these can easily cost $300-$700. If you're shooting BASE jumping air-to-air video/stills, don't get a built-in visor/shield as you'll probably never need to protect your eyes anyway. I shoot a lot of BASE jumping air-to-air and hardly ever use goggles, even when going terminal off the big walls. Another important quality is comfort. Remember that cameramen sometimes have to wear their heavy helmets for extended periods of time while filming skydives.
Camera Mounts & Enclosures - Since I video BASE jumps more than skydives, I probably over-design my camera mounts and enclosures a little. BASE jumping tends to be a bit more abusive towards electronic equipment due to the nature of the sport. I recommend that every jumper find a fast, reliable method for detaching their video, still, or both video and still cameras from their camera helmet. This is needed because cameraman will often require the use of either of these cameras in other areas such as editing or filming another project on the ground. You can build your own camera enclosures out of fiberglass and aluminum, or purchase a pre-packaged system from a helmet manufacturer. Unless you have tons of time or some serious expertise in building fiberglass or carbon composite enclosures, you're probably better off just buying a complete system. If you do decide to build them yourself, there are several designs on this webpage that may help. Typically, my baseplate is aluminum and my camera enclosures are made of fiberglass, although newer exotic materials and fabrication methods are available such as room temperature plastics and other composites.
Video and Still Cameras - Camera technology changes so much that it is difficult to recommend any particular system. However, size and weight are two very important concerns that should be scrutinized. Avoid things like bottom loading video cameras, floating lenses, proprietary cables, micro drives on still cameras (can't handle pressure changes adequately), and incompatible tape formats.
Ringsights - You can't go wrong with a Newton sight.
Still Camera Switches - More than likely, you'll be making your own still camera switch, although there are a few manucturers out there like Conceptus Camera Switches. If you are the technical type, you can purchase a remote shutter release cable for your camera, cut the wires, and solder on your own hand or mouth switch.
Video Camera Controls - If you have a compatible video camera, the best option at the time of this writing is CamEye
Accessibility Issues - It doesn't matter if you have the best looking or most expensive helmet with all the bells and whistles if you can't access the controls. Make sure your cameras are easily detachable, your buttons are easily accessible, and there are no snag points on the helmet. Cable connections should also be effortless and not require removal of other components. Think about how you'll use the helmet and do a dry run before you commit to any changes.