I really feel for you on this! The safety of our children is the most important aspect of parenting, IMHO, but getting them to comply is sometimes difficult.
Please know that your consistency in ALL areas will assist you with this issue. If you are consistent (not militant) with your rules in the house, she'll know you expect the same when you're not in the house.
For the record, the straps are meant to be VERY snug. You should be able to fit only your index and middle fingers in between your baby and the strap--and your fingers should be slightly overlapped, not side by side. Think of it as the beginning step of crossing these two fingers. These straps are made of nylon, which are meant to stretch in an accident, and therefore should not be too loose or they won't work properly. Your child will not be too uncomfortable--you're not squeezing her, just "snugging" her. : ) If you do this every time, she will be used to it and expect it. When she begins to talk, she may even let you know it's too loose!! lol
You can get more info on proper car seat applications from your DMV. If your DMV isn't cooperative, give the Dover, DE DMV a call--we have the most awesome reps there!! Nadine Hollinger and Russell Hollinger will share their great tips with you!
D.