We are huge on Car Seat Safety. Avery Rearfaced until she was 18 months old and she will be extended harnessed. MI law is harnessed until 40 pounds AND 4 years old and then you can switch to a Booster, and her car seat goes to that, but I will be buying a Graco Nautilus and will be Harnessing her until 65 pounds.
Please read through these, you could save a child's life!
Not using a safety seat consistently
"We were only going to  the grocery store ..." "He hates to ride in his car seat, so just this  once I didn't make him ..." "She was having a meltdown, so I took her  out of her seat for a minute to calm her down." Safety experts hear  these words all too often from distraught parents after tragedy has  struck. Remember, a one-time lapse can result in a lifetime of regret.
In  any case, using a safety seat consistently and correctly is the law.  All 50 states require that children up to 3 years of age (or 40 inches  tall in Kentucky) ride in car seats in private vehicles, and many have  laws requiring 
car seats or 
booster seats until a child is considerably older.
There's  good reason for that. Every year, more than 90,000 children under age 8  are injured in car crashes, and more than 1,000 are killed. In fact,  auto accidents are by far the leading cause of death for American  children.
Safety seats dramatically reduce the risk of death or  serious injury in a collision. Stephanie Tombrello, executive director  of the nonprofit passenger-safety organization 
SafetyBeltSafe U.S.A.,  urges all parents to get a safety seat that's convenient to use, and to  make buckling your child into it such a habit that you don't even have  to think about it.
Using an old or secondhand seat
That safety seat you scored  at a garage sale for a fraction of its original price may seem like a  bargain, but it could cost your child his life. The same goes for that  older-model seat your sister gave you after her child outgrew it.
Not  only are used seats unlikely to come with the manufacturer's  instructions (vital for correct installation), but they could be missing  important parts, have been involved in an accident (even unseen damage  can affect the seat's functioning), fall short of current safety  standards, or have been recalled due to faulty design. Moreover, plastic  gets brittle as it gets older, so a seat that's too old could break in a  crash.
If you must use a secondhand seat, make sure it has the  original instructions (or contact the manufacturer for a replacement  copy), has all its parts (check the manual), has never been involved in a  serious accident, and hasn't been recalled. (
Check your seat's recall status here.)
In  addition, to avoid the dangers of aging plastic, SafetyBeltSafe U.S.A.  recommends sticking with car seats that are ideally less than five years  old and definitely less than ten years old. You can usually find an  expiration date stamped somewhere on the seat.
Turning your child to face forward too soon
Children have  large heads and comparatively weak necks, so in a head-on collision (the  most common type of crash) a child's head can jerk forward suddenly and  violently, resulting in spinal injuries. For this reason, keep your  child rear-facing position as long as possible.
The  "12-months-and-20-pounds" rule that many parents cite when turning their  child forward in the car is actually the minimum size and age  requirement. If your child can still ride comfortably facing backward at  more than 12 months and 20 pounds, leave him that way. Just be sure the  seat is designed to hold children of his weight.
Moving your child out of his car seat or  booster too soon
Though 
safety-seat laws vary from state to state, all  require that children under age 3 ride in a safety seat. Experts are  unequivocal in their 
recommendations for safe riding beyond that age:
•  Your child should 
ride in a safety seat with a five-point harness until he  weighs at least 40 pounds, or until his shoulders no longer fit under  the harness straps. You can use a convertible rear- and forward-facing  car seat until your child hits 40 pounds, or the harness system of a  car-and-booster-seat combo from as little as 20 pounds up to 40 pounds.
•  Your child should 
ride in a booster seat from the time he weighs 40 pounds  and is at least 3 years old until he's 4 feet 9 inches tall and at least  8 years old.
Not installing a  safety seat correctly
A safety seat won't do its job if it's not  installed correctly. Among the most common mistakes: Not buckling the  car seat in tightly enough, and not using the right type of seat belt to  secure your child in his booster seat. Check to be sure that car seats  don't tip forward or slide from side to side more than an inch, and that  boosters are secured with a lap-and-shoulder belt.
Better yet,  use an anchoring system if you can. By law, all car seats 
and  vehicles manufactured since September 2002 must be compatible with the  LATCH system — or "lower anchors and tethers for children."
This  system combines the previously existing top tethers with lower anchors,  built into the rear of the car. Some cars built between 1999 and 2002  also have the system. Toddler/booster combo seats are required to have  both the upper and lower attachments; booster seats are not required to  work with LATCH.
Owners of earlier model cars may want to  consider having their car retrofitted with the LATCH system. Check with  your local auto dealership for information on cost and feasibility.
You  can also have your seat installation checked by a professional. 
Find  a child seat safety inspection station near you.
Not using a locking clip or using it incorrectly
If your car  is a pre-1996 model, chances are the lap-and-shoulder belts don't lock  unless you come to a sudden stop. This means you need a locking clip — a  small metal device that looks like an oversize paper clip or capital I —  to hold the seat belt (and thus your child's car seat) tight in the  event of a crash.
After you buckle your child's seat tightly in  place, see if you can move it more than an inch toward the front or  sides of the car. If you can, install the locking clip about half an  inch above the buckle — not on the other side of your child's  car seat, which pinches the shoulder and lap belts together before  they're threaded through the appropriate slot in the car seat.
If  you've misplaced the locking clip that came with your car seat, get in  touch with the manufacturer to order a replacement or buy a new one at a  store that carries car seats and other safety supplies.
Not securing your child in the seat
To  make sure the car seat harness straps are snug enough to hold your  child firmly in the event of an accident:
• Buckle your child in,  making sure the harness straps aren't twisted, and then use the  mechanism on the front of the car seat to pull the harness tight. You  shouldn't be able to pinch any harness fabric between your fingers.
•  Slide the plastic retainer clip that holds the two straps together up  to armpit level before securing it. If the clip is too low, your child  could be ejected from his seat in a crash.
Not  buckling a car seat into the car
Believe it or not, many parents  who are cited for car seat violations have their child buckled into a  car seat — but have not buckled the car seat to the car. This  may be the result of confusion about how the seats work or just of  switching a seat from one car to another on a hectic morning.
To  avoid this mistake, when you're putting your child in his seat,  double-check to be sure that the seat is buckled tightly to the car.
Holding your child on your lap
It's tempting to lift your  child out of the car seat and hold him in your arms when he's having a  tantrum after hours on the road, or when you're making a quick dash from  one place to another with a gaggle of friends and it's easier for  everyone to pile into the same vehicle than to take separate cars.
This  might seem safe enough. After all, you'd hold your child tight  if anything happened, right? But the truth is that even if you're  belted in, your child could be ripped from your arms by the force of a  collision. And if you manage to get the seat belt around both of you,  your weight could actually crush your child to death.
So as much  as your child may scream — and as inconvenient as taking your own car is  when the two of you could just hop into someone else's — never  let your child ride in a moving car unless he's safely strapped into an  age-appropriate, correctly installed car seat or booster.
Letting two kids share one seat belt
Don't  do it. Crash tests have shown that when two children ride buckled into  one seat belt, in an accident their heads can knock together with  potentially fatal force.
Letting  your child ride in the front seat
Although your child may whine  and plead to ride in the front seat with you, the backseat is by far the  safest place for him. The National Highway Traffic Safety  Administration (NHTSA) recommends that all children under age 13 
ride in the backseat every time they get in the car.
Other  tips on safe riding positions:
• If possible, buckle your child  into the middle of the backseat, where he's best protected from  side-impact collisions. (Of course, if you have more than one child only  one can ride in the middle!) If your child rides in a booster seat and  your car has only a lap belt in the center back (which is fine for car  seats but should never be used on its own with a booster seat), position  your child's booster on either the right or left side of the backseat  and buckle him in with the car's lap and shoulder belts.
• If you  have a passenger-side 
air bag in your car, that's all the more reason to keep  your child in the back. Air bags are designed to reduce injuries in  adults, but they can cause serious head and neck injuries to children  when they inflate, especially if the child is in a rear-facing car seat.  To find out whether your car has air bags, look for a warning label on  the sun visor or the letters SRS or SIR embossed on the dashboard, or  check your vehicle owner's manual.
• If putting your child in the  front seat is your only option (for example, if the backseat is full or  your car is a two-seater), check to see whether your car's air bag has  an on-off switch. If so, turn it off. If not, have one installed. The  NHTSA maintains 
a list of companies that install air bag on-off switches.
If  you can't turn your passenger-side air bag off and you need to  transport a child in the front seat, put your biggest or tallest  passenger in the front, in an appropriate safety seat, and move the seat  as far back from the dashboard as possible.
Find  more car seat buying advice from the American Academy of Pediatrics.