This blogger has given some great tips on car seat mistakes you might be making that you haven't even thought of yet.
I'd like to add a few:
The Chest clip- is just what it's called it's not called a belly button clip or a neck clip! Put it at chest/armpit level.
The Straps should be tight- but not so tight that they hurt. If you can slip just your finger underneath that should do it
Check out http://www.car-safety.org/faq.html for some great tips.
Be extremely cautious when using a "used/yard sale" car seat. Only use it if you are confident that it has a clean "record". Or better yet, just don't! You never know whether it's been in an accident or abused somehow. I've even heard of someone who bought a seat on craigslist and the seller had altered the date of manufacture from a 2003 to a 2008! The buyer only found out the seat was expired when they went to register the serial number. Even if it isn't malicious, people have different ideas of what counts as an "accident". Check your car seat manufacturer's website to see when they require a seat to be replaced. (The only used car seat I would allow my children to ride in is Mommy J's because I know if she got into an accident I'd probably be one of the first people she'd tell and I know when all of her car seats expire anyway.)
Most car seats have a 6 year expiration (a few have 7-9 years). Plastics do degrade. Okay so maybe they won't biodegrade but if you have ever left plastic patio furniture out for 6 years in the elements it gets brittle. Car seats are subjected to drastic temperature changes in the car- freezing to super hot.
Try not to check car seats at the Airplane. It's best if you can purchase a seat for your child and use the car seat on the airplane. Some car seat manufacturers even consider them to be "crashed" if they have been checked on an airplane. There are some shocking youtube videos that portray the rough handling they are subjected to not to mention shifting around with heavy luggage.
Finally- If you find out you've been making a car seat mistake all along, don't beat yourself up. Fix it. Learn from it and move on.
Magic beans in Wellesly, MA is hosting a Car Seat Check August 28th 2011 from 2-5pm. There's limited space so sign up!
or you can try to find a local event at http://www.safekids.org