I've posted this before. People suggested me to use car safety locks which I am doing and special additional belts. My dh thinks that I should not put additional belts since in case of accident I will have hard time to take her out quickly.
Anyway very often she gets out from her car seat during driving and just standing on a back seat holding her hands on a front seat. I think she could fly out the window even if I have to do emergency stop and she doesn't understand any conviencing or shouting.
What kind of seat is she in? Is it a booster seat that just positions the car's seatbelt across her or is it a seat with a harness in the front? Have you had the seat for long?
If you haven't had the seat long maybe you should take it back to the store and explain the issue you are having. Perhaps they will let you swap it for one that is harder for her to get out of. I know some stores have display models and with thier permission you can try seating her in them and getting a feel for what her little fingers can do.
My sister has the same problem with my neice. She finally found one that buckles between her legs and then across her chest. She can't press the button between her legs because of it's position so now she is unable to get out.
1. Pick a really fun place to go that your daughter wouldn't want to miss (this will work best if she has a friend, sibling or cousin that could also get to go), then right before you go, you say "I'm sorry, but you always unbuckle your seat and get out, so you don't get to go today because that isn't safe. When you start to listen and stay in your seat, you can go next time." Then GO and leave her home. A couple times of this, it WILL work.
2. I'm sure some people are going to have a wig out when I say this, but if you must take her in the car with you and she just refuses to stay buckled, put mittens on her and duct tape her hands together. It won't physically hurt her, don't make a giant fuss about it, just tell her you want her safe and it's the law that she be buckled. Then when you get to your destination, untape her, remove the mittens, again making no fuss. If she throws a tantrum during the drive, do not talk to her, do not cajole or bribe, just get wherever you're going, remove the mittens and go on about your business. She'll get the clue that you're very serious about her safety. You aren't yelling, you aren't spanking, you're keeping her safe.
3. If she gets unbuckled, immediately and safely pull over or off at the nearest exit, get out of the car, tell her she may not be unbuckled in the car, then give her a 4 minute time out right on the side of the road in the parking lot, whatever. Once again, don't argue with her, don't bribe or coddle and don't talk to her during the time out.
I had the same exact problem with my 3 year old daughter. The way we coped was by providing her distracting things to do. We gave her crayons and paper to color, books to read, her own little cd player to listen too, dolls, all kinds of things to keep her occupied during car trips. If she undid her buckles anyways, which she sometimes did and still (rarely) does, we have turned the car around and gone home. We explained to her why it was important to stay buckled and that everytime she unbuckled herself we we're going to cut the trip short and go right home. Be consistent and follow through and she should stop.