Among the easiest mistakes to avoid is to just make sure that your child is in the correct car seat for his age and that he is facing the right direction.

• Infants should be in a rear facing infant only seat or convertible seat until they are 1 year old AND twenty pounds. Children who reach twenty pounds before their first birthday still need to face backwards and can be moved into a rear facing convertible seat. Smaller infants who don't reach 20 pounds until after their first birthday should also continue to face backwards. This is more a minimum though. Many people advocate continuing to sit toddlers rear facing in a convertible seat until they outgrow it for added safety.
• After they are twenty pounds and have passed their first birthday, toddlers can use a forward facing car seat (either a convertible, combination or forward facing seat) until they are about 40 pounds.
• Children over forty pounds should be placed into a belt positioning booster seat (either a combination seat or booster seat) and they will usually stay in it until they are at least 8 years old.
• You should not use your car's regular seat belts until they fit correctly when your child is about 80 pounds and is 4ft 9 inches tall. Remember that your child will not be ready to use regular seat belts until the shoulder strap fits across his shoulder and not his neck, and the lap belt fits across his hips and not his stomach.
• All children under 12 years of age should be placed in the back seat of the car, especially if you have passenger side air bags.

Once you have the right seat, it is easy to make mistakes by not securing the seat correctly in your car or not securing your child correctly in the seat. Common mistakes when using a car seat include:

• having harness straps too loose or in the wrong position
• having a harness chest clip in the wrong position
• not locking the seat belt properly with a locking clip, seat belt retractor or locking latchplate. Keep in mind that newer seat belt systems have a built-in locking mechanism.
• not securing the car seat correctly, by either using the wrong seat belt path or not making the seat belts tight enough
• placing an infant seat in the path of an air bag.

Other mistakes to avoid depend on what type of seat you are using. In addition to following these tips, you should also read your car seat's instructions. A recent study showed that many of these instructions are difficult to understand by many parents, so if you don't understand what you are doing, either call the manufacturer or go to a car seat inspection station to see if you are using your seat correctly.

To make installation even easier, consider getting a car seat and car that has the new LATCH system, which doesn't need to use your car's set belts. LATCH tether anchors can also be added to older cars.

When using an infant seat , make sure that:

• the harness chest clip is correctly positioned at your child's armpit level so that the shoulder straps will be in the correct position
• the harness straps are snug and straight
• rear-facing harness straps are positioned at, or slightly below, your child's shoulders
• the seat reclines at about a 45 degree angle
• you never place an infant in a rear-facing child restraint in the front seat of a car with a passenger side air bag

When using a rear facing convertible seat , make sure that:

• harness straps on rear-facing seats are positioned at, or slightly below, your child's shoulders
• the harness chest clip is in the correct location at your child's armpit level
• the harness straps are snug and straight
• the seat reclines at about a 45 degree angle

When using a forward facing convertible seat , make sure that:

• harness straps on forward-facing restraints are positioned at, or slightly above, your child's shoulders. You should be using the top set of harness slots for convertible child safety seats.
• the harness straps are snug and straight
• the harness chest clip is positioned at your child's mid-chest or armpit area.

When using a forward facing combination seat , make sure that:

• harness straps whould be positioned at, or slightly above, your child's shoulders.
• at 40 pounds, you remove the harness straps and use your car's lap/shoulder belt, especially if the harness straps are below the child's shoulders.
• you stop using a shield booster once your child is 40 pound

When using a belt-positioning booster seat , make sure that:

• you always use the lap/shoulder belt combination with a belt-positioning booster. Never use a lap belt only. This includes no back and high back booster seats.
• the shoulder belt rests snugly across chest, rests on shoulder; and should NEVER be placed under the arm or behind the back. 
• the lap-belt should rest low, across the lap/upper thigh area, and not across the stomach.

You should also avoid using a car seat that has:

• been recalled
• involved in a crash
• is more than 10 years old (or depending on the manufacturer, more than 5-6 years old)
• doesn't have a label with the date it was manufactured and the seat name or model number
• doesn't have instructions
• is missing parts or has cracks in the frame