Dumbbells. Heavy shrugs. Lateral raises, front raises, rear raises, supersetted. Seated dumbbell presses, no back support. There are also special head harnesses for working the neck muscles, but I've never used one. Like anything else, a stronger muscle tires more slowly when loaded. Just using one's head for load isn't enough to generate much strength. There aren't any fast, coasting descents around here longer than about 16 miles, so that's my experience.
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