Skiers and snowboarders usually suffer head and spine injuries in high speed collisions or falls. Although there is good evidence that helmets would prevent many problems, they are currently only worn by racers.
There are two types of head injury, internal and external:
Internal: These can involve the skull, the blood vessels within the skull, even the brain. A concussion is a typical example of this.
External: These are usually scalp injuries - they look spectacular, with lots of blood, but are not usually too serious.
Internal damage that is of more concern, with doctors recommending that those who have suffered a mild concussion do not participate in contact sports for at least three weeks as a further blow to the head within a few days or weeks can cause serious brain damage.
Spinal injuries vary greatly in severity, from bruising to complete shearing of the spine. The higher up the site of the injury, the greater the chance of paralysis or death.
There is little that can be done in the way of conditioning to avoid directly avoid these injuries - however, good conditioning means that falls are less likely in the first place.