The seat must be designed for use as a booster seat.
Be sure you check the labels and manual for the seat to find out whether the seat can be used as a booster.  Booster seats that are designed for use at the table are not for use in vehicles.  All boosters seats should have a label indicating that it meets motor vehicle safety standards.

The seat must fit well in the vehicle.
If the seat tips when going around corners or slides around on the vehicle seat, it is not compatible with your vehicle.  A compatible seat should stay upright when occupied and the base should remain against the vehicle seat back at all times.

The child should have adequate head support.
A backless booster can be used in seating positions where the seat backs are higher than the child's head or head restraints are available.  A high back booster must be used if the tops of the child's ears are higher than the seat back or head restraint.  If the top of the child's ears go above the top of the booster seat, move to a backless booster if head restraints are available in the vehicle.  Move to a taller high back booster if there are no head restraints available.

The lap and shoulder belt must both be used.
There are no booster seats that can be used with just a lap belt, with the exception of shield boosters which are not recommended.  The lap and shoulder belt must both be used with a booster seat.  Using only the lap belt risks head and neck injury.  Using the shoulder belt risks submarining (sliding under the belt) and ejection.

The seat belts must fit the child correctly.
The lap belt should fit snugly over the child's upper thighs; the lap belt should not ride up onto the stomach.  The shoulder belt should cross the shoulder comfortably without slipping onto the arm or rubbing against the neck.  The "arms" on the booster base and the shoulder belt guides should help keep the seat belts in the proper place.

The back seat is the safest place.
Everyone would be safer in the backseat - in the middle, if possible. The back seat is safest because it is farther from any point of impact. A front seat passenger is 30% more likely to be injured or killed than a rear-seat passenger.  Use the back seat position that offers a lap/shoulder belt and a good fit. 

*NOTE* In mini-vans, the safest position may be in the MIDDLE seat, as the rear has less "cargo space" to absorb a rear impact.

Avoid using add-on products 
Anything that did not come in the box with the seat could potentially put your child at risk. Adding seat belt covers could cause the the belt to be positioned incorrectly. Also, never use an aftermarket belt positioning device in place of a booster seat.

The child must fit properly in the seat  There are several conditions that must be met for a child to fit correctly in the seat.

A child is too small for the seat if:

• The child weighs less than 40 lbs. A seat with a full harness is safest and should be used until the child reaches 40 lbs.
• The seat is too wide or too long.

A child is too large for the seat if:

• The child weighs more than the seat's upper weight limit.
• The top of the child's ears are above the top of the booster seat (high back booster) or vehicle seat back (low back booster).
• The seat is too narrow or too short.

Used seats may be dangerous
Never use seat that is damaged, under recall*, over 5-6 years old, or has an unknown history. Check out Not New? for more information on any seat that is not brand new before using it.

*Some recalls do not affect the safety of the seat, and the seat may safely be used to transport your child until the problem is fixed.  You will need to contact the manufacturer to find out whether any recalls on your seat must be fixed before using it for transporting your child.