Make sure the belt stays snug and well below your abdomen. Adjust Your Shoulder Belt . . . Position your shoulder belt over the top part of your shoulder and across your chest. That distributes crash forces over a large area of your rib cage and protects your soft organs underneath. Make sure the shoulder belt lies comfortably snug against your body.
Some shoulder belts work like window shades.
Each time you lean forward, even a little, and then back the shoulder belt loosens and stays that way unless it's tugged on. There should never be more than one inch of slack between you and your shoulder belt. Get into the habit of check your belt often. Don't tuck your shoulder belt underneat your arm. In a crash, this puts pressure on your ribs, which are thin and easily broken.