My biggest pet peeve as a parent is seeing a child incorrectly buckled in their car seat.  A car seat is specially designed to protect an infant/toddler in the event that you are involved in a car accident. In order for the car seat to properly protect your child, the buckle needs to go across your child and should be even with their arm pits.  I will post an example below of my toddler incorrectly buckled and correctly buckled in his car seat.  The best way to determine if the car seat straps are tight enough to protect your child, you should only be able to slide two fingers in between the straps and your child.

With winter quickly approaching, many parents feel the need to bundle their child up in thick clothing.  However, when they are in their car seats it is best to avoid bulky, thick clothing.  Thick and bulky clothing can prevent the car seat from properly securing your child.  Instead, put a light weight jacket on them and place a blanket over them.