Matilda used to crack it everytime we got into the car. She would stiffen up her body straight and I couldn't get the seatbelt on. I used a few techniques....
1.) offer her a fruit stick if she sat (bribery...)
2.) hold her in the seat until she settled and then give her a kiss and praise her.
3.) wait for her to climb in the seat herself and make a game out of it... how fast can you get in the seat?

Goodluck!! & try most of all to let her know that you don't really care about it.