The Position of the harness.
It's the one thing we might not think of to much about, but is very important to the safety of your child in their car seat. The harness chest clip should be fastened and positioned at your child’s mid-chest or armpit level, which keeps the shoulder straps from slipping. Harness straps should be snug and untwisted. Nothing should be between your baby and the harness. Do a test: If a harness is properly snug, you shouldn’t be able to insert more than one of your fingers behind it or easily pinch any slack in the belt. Position rear-facing harness straps in the slot positions at, or slightly below, your child’s shoulders. On forward-facing and convertible seats, which come into play after your baby’s first birthday and the 20-pound benchmark, harness straps should be positioned at, or slightly above, your child’s shoulders.
Take off the wraps.
If you need to keep your baby warm, place blankets or thick coats over him/her after you strap her into an infant or convertible car seat. Don’t wrap your baby up in a blanket, thick coat or other bulky garment and then strap her into a restraint system. That may prevent the restraint system from working properly.
You can read more on this on ConsumerReports.org.