How to wear a seat belt correctly
The lap belt goes low and snug across the hips, not across the soft part of the abdomen. The shoulder belt crosses the center of the chest and shoulder, not under the arm or behind the back. A belt worn under the arm can cause severe internal injury in a crash; a shoulder belt behind the back provides almost no protection at all.
Primary versus secondary enforcement
In most states, seat belt violations are primary offenses, meaning an officer can stop you for the violation alone. In a few states they are secondary, meaning a stop must be for another reason first. Either way, the fine is small but the consequences of not wearing one are not.
Pregnant drivers and special cases
Pregnant drivers should wear the lap belt below the belly and the shoulder belt across the chest, not over the abdomen. Drivers with medical conditions that make a seat belt difficult should consult a doctor about modified belt configurations rather than skipping the belt.