Only forward movement of the spectacle relative to the face will cause the spectacle to slip down.
In a rigid body, where the points of contact between the spectacle and the face is fixed, the spectacle will not slip. For the spectacle to slip down the nose bridge, it will have to move forward and away from the face. This is possible as the spectacle is just resting on the nose bridge and the ear and there are always movement around the points of contact. These movement may come from moving of the head or facial muscle movements.
Heavier lens have a greater tendency to slip down due to weight.
Most opticians take pride in customizing your spectacles to give you the perfect fit which includes minimal spectacle slipping. Unfortunately, slipping spectacles is still one of the most common complains. For prescription spectacle, the lens is usually the heaviest part. When looking straight, weight of the spectacles are on your nose pad which is resting on your nose-bridge. However, movement of the facial muscles and the head will often cause the spectacle to slide forward and downward due to the weight of the lens and its location at the end of the spectacle. To address this problem, it is important to understand how the parts of the spectacle work on the skull to support it.
Frame that is too narrow will push the spectacle away from the face causing it to slip down.
Minimal slipping of the spectacles starts from the spectacle frame selection. A frame that is too narrow for the face will tend to push the spectacles outwards as its temple presses against the skull. When this happens, the nose pad will be pushed away from the nose bridge which causes the spectacle to dip down. In a matching spectacle, its temple should be resting on the ears with minimal pressure on the wearer's temple.
The bend on the spectacle temples is too loose.
In an ideal situation, the bend on the spectacale temples are perfectly contoured behind the ears so that it locks the spectacles in place preventing any forward movement. Without any forward movement, the spectacle will not slip as the nose pad will be fixed in the same position on the nose bridge. However, this will exert a constant pressure on the back of the ear leading to soreness. The spectacle temple is usually rigid and it is difficult to bend it to the perfect contour such that it just touches the back of the ear. Therefore, there is usually a little 'wiggle' space between the bend and the back of the ears and if this space is too large, the spectacle will slip down.
The spectacle temples will also be contoured such that it is applying a light pressure against the skull, enough to generate the holding friction but not enough to cause soreness or redness. For most spectacle design, the temple towards the tip are flat such that the force on the skin can be spread over a larger area. This is to minimize the pressure on the skin. Note that if the pressure is above an individual's threshold, there will be discomfort at the point of contact.
Nose pad surface that is not in full contact with nose bridge.
The nose pad provided the second frictional forces that keeps the spectacle in place. As the skin on the nose bridge is relatively soft, any pressure point on it from the nose pad will cause slight depression to form. This slight depression on the skin will allow accumulation of sweat and oil which reduces friction. When there are movement from the face, there is insufficient friction to keep the spectacle in place causing it to slip down.
Insufficient friction at nose pad and temple.
Friction at the nose pad and temple are needed to ensure that the spectacle stays in place despite movement of the head and facial muscles. Friction is affected by the amount of force applied normal to the surface and the friction coefficient between the contact surface. Since our skin has a low tolerance for pressure, the amount of force that can be applied is limited although a larger force can still be applied if it can be spread out over a larger contact surface to reduce the pressure. The material used for the temple and the nose pad may have a low friction coefficient and this will also reduce the friction.
With the right level of applied force and selection of material, friction may still be reduced due to external influence. Perspiration and facial oil acts as lubricants between the contact surfaces and this will reduce the friction. Hair that comes between the spectacle temple and the skin will also reduce friction.