The combination of these points of adjustment can make the glasses feel comfortable and relaxed on your face, but it requires all points to be worked together and in a choreographed sequence in order for the operation not to become “Rubik Cube” like.
The first thing to do is to ensure that all points of contact are tight and firmly connected – so check each screw to make certain that it is tight. This will ensure that any further adjustments not only work correctly, but also stay in place for a reasonable amount of time. The usual symptom of “having a screw loose” is that the sides of the frame become loose and floppy on your face – so that is always a good place to start.
If the glasses are slipping down on your face, it is likely that the sides of the frame are not symmetrical and one or both need some adjustment. Place the glasses face down on a white towel or sheet and looking straight down at the lens along the side frames, see if they are directly perpendicular and orthogonal to the front frame. If one or both are not then prepare to adjust.
All frames have some metal running down the centre of them, and it is this that needs to be bent back into shape – that’s one of its purposes ! The difficulty is that the metal is often coated or surrounded n plastic or acetate that is prone to cracking or breaking when bent.
The best way to approach this dilemma is to heat the sides of the frames with a warm air source – a domestic hairdryer works very well – and then adjust each side alternatively in small increments until they have achieved the regularity of the frame.
In the case of metal frames, the glasses can be made to sit higher or lower on the face by adjusting the nosepads inwards or outwards. Again, take care and do each one alternatively and incrementally so that you can measure the result from each tweak and adjust accordingly. Plastic frames generally do not have nosepads.
If the glasses are sitting on your face comfortably, but are not straight or level on your face, it is likely that one side is bent up or down relative to the other side. To best remedy this, firmly grasp at the hinge – and then gently bend. Be careful however, as considerable force is sometimes required, and this is a natural weak point in spectacles, and so they can break here quite easily. Be firm with positive force rather than quick jerky movements.
Found this article at Fashion Specs Direct