Never use a booster with only a lap belt. Choose a high-backed model if your child often falls asleep in the car or sometimes rides in a car with a low seatback. If the child's head is supported by the vehicle seat, consider using a backless booster. It may fit the vehicle seat better, and there are special head-positioning pillows for improved comfort. Look for a wide or adjustable base if the child is short and chunky.
Most boosters have a belt-positioning strap or clip for improved comfort.
The shoulder belt should be placed at the center of the shoulder, not near the top of the arm. Most children need to use a belt-positioning booster until they are at least eight years old, depending on the child's height and where the safety belt is attached to the car. The child should continue using a booster until he or she can sit all the way back against the vehicle seat with the knees bending comfortably at the edge of the seat cushion. Otherwise, the child may slouch or slide forward to the edge of the seat, which is very dangerous. The lap belt should cross the hips or tops of the thighs, and the shoulder belt should be centered on the shoulder and chest.