In my glasses sales days, clients would come in asking us to tighten the screws on their sunglasses and glasses to snug up the loose frame that now slid down their nose. We would tell them that that the screws weren't the problem...
When you push your (sun)glasses on your head, you stress the frame by putting pressure on the temples, which in turn places pressure on the frame front. The frame front is gradually warped into a flatter shape rather than the curved shape of its manufacturing.
If your sunglasses are of poor quality plastic, the frame itself wants to return to a flat shape (the original state of plastic). If your sunglasses are well made, a skilled optician will heat the frame front and coax it back into curvature to move the frame closer to the temples, and thus "tighten" the frame altogether.
If your sunglasses are of cheap metal, trying to bend the frame back into curvature will either snap the metal or reveal the true material colour beneath the paint. Higher quality, non-painted metals will bend back without much damage by a professional hand.
If you just continually tighten the screws, you'll strip the threads on the screws and still have loose frames.
Best to put them in their case, or just fold them up and hang them off the neck of your deep-vee.