Hi...
Lots of good ideas here but I thought of another that my friend suggested. If it seems like your daughter is about to get willful and wrestle you...try jokingly putting her in sideways/upsidedown/backwards, etc. I have done this a couple times and so I'll put her in sideways and be like "Ok THIS is how kids sit in their carseats!" and my daughter will say "Noooo Mama!" and then I'll try putting her in upside down and say "Oh, then it must be THIS way!" and she'll say "Noooo Mama!" and if I am lucky she will sit right down the correct way and say "THIS way MAMA!!!!" all proud of herself.
I think toddlers like thinking they have pulled something over on us...but in this situation they are SO wrong: Mama: 10 points, toddler zero! :)
In the end, when they wrestle me, I try what I can to get them in there in a friendly manner but when all else fails, I too will resort to stern mommy voice and some wrestling. After all, it is for their safety. I do try to distract too by talking about what we'll do when they're in their seats (have some water, drive to Grandma's, listen to music". I get sweaty and frustrated but in the end I know I am keeping them safe and I have to just keep remembering I am the boss. :)
Good luck!!!
-S.