5 Key Mistakes Parents Make with Car Seats

• Using a defective car seat. Don't buy a used seat; you don't know its history. Avoid old ones (more than 10 years old), especially with missing parts or cracks. And never use seats that are missing a label or instructions, have been recalled, or were in a crash.
• Using a forward-facing car seat too soon. Don’t hurry. Keep your child in the rear-facing seat until he or she grows out of it. Your child seat user guide will tell you the maximum weight and height of a child for that seat. If your child grows out of the rear-facing seat, there may be another model that will still fit your child. Some rear-facing car seats are made for children up to 20 kg (45 lbs)! Use an age appropriate car seat that meets government approved standards and follow the manufacturer’s installation instructions.

• Installing the car seat incorrectly. It shouldn't move more than 1 inch. You don't need a seatbelt to secure the car seat if both it and your car come with LATCH (Lower Anchors and Tethers for Children). But still make sure it's tight, and never put the car seat in the path of an airbag.
• Securing the harness straps incorrectly. They should always be snug and straight. For rear-facing car seats, use the two lower slots and strap the harness slightly at or below the shoulders. For forward-facing, use the top slot and strap slightly at or above the shoulders.
• Positioning the chest clip incorrectly. Snap the chest clip at armpit level for rear-facing car seats, and at mid-chest or armpit level for forward-facing ones.

 Online Source:

Car Safety Seats: A Guide for Families 2007, American Academy of Pediatrics