How to prepare for a trip to the Half-Dome in Yosemite National Park?

Indeed, the prospect of someone else falling and leading to a 'domino effect' is frightening. When I was there, I saw someone drop a water bottle. It bounced a few times and then plunged thousands of feet down the granite face.

The National Park Service has recognized that the Half Dome ascent has become too crowded, and recently began requiring permits for the hike. It appears that they are currently considering what scheme to implement for the coming summer. The Half Dome Permits website currently (Dec 2011) says:

The Half Dome cables are down for the season. A long-term plan dealing with management of and access to the Half Dome cables will be released shortly.

If you are concerned for your safety, you could wear a climbing harness. Attach a length of webbing, and use a caribiner (or two). Clip onto the cable.

The clothing of the people in the photo looks reasonably appropriate. It will be warm/hot during the day while you are exerting yourself, but will get cold at night, or when you stop for a break. Bring a warm fleece pullover for these times. As when hiking anywhere, you should bring sufficient clothing so that you could spend the night without excessive discomfort. Bring a poncho or other rain gear. If a thunderstorm occurs, descend immediately to below the tree line to avoid the risk of being struck by lightning.

The hike is not technical, but if you are not accustomed to the altitude, it can be difficult and slow-going; unless you are in very good shape, you will probably need to stop frequently to catch your breath. Start the hike at dawn. Bring a flashlight in case you have to make your way back at night. Bring plenty of water, a good lunch, and some snacks.