[Paul F]

Shorter hair - if you have longer hair, the hair in the back just slips up and off goes the hat. When I shortened my hair, the cap stayed on much easier

Deeper cap - Many caps go cheap on the material and are somewhat shallow. The back of the cap ends up on the upper curve of the head and it can't grab onto anything. Sort of like a fat guy trying to keep pants on with no hips. I buy plain caps in various colors at K-Mart. They are nice deep caps that sit lower on the back of the head and stay there.

Forget the cap - Buy some motorcycle sunglasses. They come with a removeable foam gasket that blocks the sun in the gap between the rim of the glasses and your head. Plus they keep the wind out. Nice.