Headrest Location The headrest needs to be positioned as high as possible, but not obstruct the driver's vision out of the driver's and passenger side window. Remember, you only need to see out of the passenger window and don't need to see the bottom door bar. This positioning helps control the head motion. If the headrest is too low, the head will tend to roll over it in an impact. Newer headrests are being made to allow much better vision side-to-side also, but by wearing a head-and-neck restraint, you won't be able to turn your head as much as without one anyway.
Another important part of the headrest, recently brought to mind when I was monitoring a sled test, is a durable cover. A strong cover (vinyl) that will help the helmet slide instead of dig into the inner foam should be installed.
The choice of a long or short headrest is a personal one, but if you do run a short headrest (meaning less than 12 to 14 inches from the back foam), you should run a supplemental net restraint system. The net should be run inside the headrest's foam and be positioned to prevent the head from getting under or behind the headrest. The main idea of the nets is guidance of your head during an impact. Nets should never be used instead of a proper headrest. The net should also have a release mechanism to allow for a quick escape.
Shoulder supports need to wrap around the shoulder. They do not need to touch the occupant, but the closer you are comfortable wearing them, the better they will perform. Shoulders should be stiffer than the headrest because they control a larger body mass (torso vs. head). They need to be connected to the headrest to ensure they both move together evenly. The stiffer shoulders need to have a load-limiting foam in them also. A minimum of 1.5 inches of foam needs to be run in shoulders that can support loads of 2,000 pounds and up with less than 1-inch deflection.
Once the full headrest and shoulders are mounted in the car, the driver needs to spend some time inside the car getting used to the new driver's compartment environment. You will need to plan out and practice escape routes to the passenger side to become familiar with them. Clearly, it's not a good idea to try to exit out the right-side window for the first time after a hard impact when the car is upside-down and on fire.
It is your safety system, be familiar and comfortable with it.