DO make sure that the harness-retainer clip -- if there is one on your car seat -- is at the level of your child's armpits. If the clip is in the wrong place, the straps could slide off.
DO put an infant in a rear-facing car seat at a 45-degree angle. If it's too flat, your baby could slip down; if it's too upright, his head could flop forward and squeeze his airway. (Use a rolled-up towel under the foot of the seat if necessary.)
DO put a tightly rolled-up blanket alongside your baby's body if he's small. (Strap him in first so the straps stay taut.)
DO take your car to a local car-seat inspection site so a professional can check the installation. To find a location near you, go to seatcheck.org.
DO put your child in a booster seat when she's too heavy for her car seat, and insist she use it until she's 8 years old or 57 inches tall. A new study found that high-back boosters are safer in side-impact crashes than backless ones.
DO place the harness straps in the slots below your baby's shoulders when her seat is rear-facing.
DO make sure the harness straps are above your child's shoulders in a forward-facing seat to give her the most protection in a crash.
DO choose a car seat with a five-point harness, which will hold your child.
DON'T put an infant in a forward-facing car seat. A baby should ride in a rear-facing car seat until he's at least 1 year old and weighs 20 pounds. If your baby is less than 1 year but weighs more than 20 pounds, put him in a convertible rear-facing car seat.
DON'T forget to tighten the safety-belt attachments when installing your seat. The seat is too loose if you can wiggle it more than an inch from side to side.
DON'T put your car seat or booster in the front seat. Children must be at least 13 before they can ride in front.
DON'T forget to check that the tethers are attached correctly if your car uses the LATCH system. For a rear-facing seat, you need to use only the lower attachments; for a forward-facing seat, also use the top tether. Be sure to tighten all straps.
DON'T ignore height guidelines for your rear-facing infant seat. A baby's head should be at least an inch below the top of the car seat's shell. If it isn't, it's time to switch to a rear-facing convertible seat.
DON'T leave the harness straps loose. The straps are tight enough when you can't pinch any harness fabric between your fingers and you can fit only one finger underneath.
Sources: Flaura K. Winston, M.D., Ph.D., scientific director of pediatric trauma at the Children’s Hospital of Philadelphia; National Highway Traffic Safety Administration; American Academy of Pediatrics; Stephanie Tombrello, executive director of SafetyBeltSafe U.S.A.; and Partners for Child Passenger Safety.