Hinge Adjustment
Over time, sunglass hinges wear and the screws work themselves out. Loose hinges generally imply a looser fit and a higher probability the sunglasses will fall off your head or slip when you are wearing them.
Hinges should be tight enough that a gentle shake of the sunglasses does not cause the arms to move. They should also be loose enough that they don't bend or strain when opening and closing.
To tighten your sunglasses, determine if you have phillips head or flat head screws. It's important to use the correct screwdriver. Having a few quality screwdrivers around the house will pay for itself over time.
Grip the sunglasses firmly and tighten carefully. About 5 degrees is normally all it takes. After each turn, do the shake test again until the arms don't move, but close smoothly.
Once both arms are adjusted, dab the screw head (and screw bottom if exposed) with clear nail polish. This will help to protect it.