I found this great car seat safety post over at Treading Raging Waters. I highly suggest checking it out to make sure you are truly using your child's car seat in the safest manner possible. There is a lot of good info and facts about they whys of how you use a car seat like why does the chest clip need to go on the chest.
I also really like this link at Daily Momtra. It has pictures of the proper use of the car seat in regards to chest clips, harness positioning, etc.
This is a car seat pop quiz that has lots of info as well.