Eyewear slips down only when its position shift forward of your face
In an ideal situation, your eyewear temple/arm will prevent your eyewear from moving forward relative to your face. However, to ensure user comfort, any pressure from the temple needs to be low and the resultant force may not be sufficient to keep the eyewear in place.
A physical barrier that stops the eyewear from moving forward
Most long spectacle hook provides anchorage by acting as a stopper behind the ear. The spectacle hook is able to extend down the back of the ear and adjusted such that it is just touching the ear. This spectacle hook becomes a physical barrier that prevents the eyewear from moving forward.
Increase friction by better distribution of higher force over a wider contact area
Contoured hook that wraps round the of the ear.
Depending on the design of the spectacle hook and its position on the user's ears, it is possible to increase the force while reducing the pressure to the skin. If the area of contact between the spectacle hook and the skin can be increased, a larger force may be applied without causing discomfort. This is typically achieved by having the hook to contour around the back of the ear, use of softer or more flexible material.
Improve anchorage by using material of optimum (not excessive) friction coefficient
"sticky" material adhering to the skin when pressed on.
Friction is the product of normal
force and the friction coefficient between the contact surfaces. While greater force will result in higher pressure under the same contact area, another way to increase friction is using a material with a larger friction coefficient. Material that has a rubbery feel against the skin will have a higher friction coefficient compared to a smooth and slippery surface. However, if the friction coefficient is too high, it would be uncomfortable to the skin. Therefore, a material that feels "sticky" is not recommended. A simple way to test is to use your hand to press the material on a metal or paper surface. Lift up your hand and if it sticks to your skin, it fails.