At that age, my son started to get to be fascinated with the chest clip. (He still is, a few months later.) So if he gets into his seat without monkeying around, then he gets to buckle the chest clip. Otherwise I do it for him (and he gets really mad).
He also likes to ride with stuffed animals, so using one of those as a reward works well for him. Another possible reward might be listening to a CD that she likes (we can't do this b/c our CD player in the car is broken).
It takes a little more effort, but she might respond well to you really talking up how she is such a big girl and can climb into her seat and sit right down and do such a great job, etc, etc. Start the pep talk as you're going out to the car, tell her what you expect of her and how you know she can do such a great job, praise her as she's climbing in, and keep it really positive (Yay! you did it, wonderful! What an obedient girl!). That sort of technique worked great with my dd (who, coincidentally, is named Grace). DS is not quite as motivated by praise.