What causes 'brake fade' on a motorcycle, and how can it be prevented during long descents?

1. Excessive heat buildup that reduces the friction coefficient of brake pads; prevent it by using engine braking.
2. Air entering the brake lines; prevent it by squeezing the lever faster.
3. Worn out brake pads; prevent it by applying lubricant to the calipers.
4. Water on the brake discs; prevent it by keeping the brakes applied constantly.