I forgot it on the last post so I'll just make a new one! The most common misuse of car seats is just simply It's not tight enough! If you car seat moves more than 1" at the belt path, it's too loose. Try putting weight (I put my knee into the car seat) in the car seat while you are tightening the seat belt / LATCH strap. This will compress the seat and help you get a tighter fit.
Here are some great car seat installation video's!