My son gets terribly motion sickness in the car, (he's now 5)to the point where he would throw up if we were traveling in the rain. I use the elastic wrist bands on him, I think they help. (I got them at Walgreens, I think).
I also recommend letting her snack the whole trip, and lollipops in your purse as an emergency measure. (they can help with ear popping) I have three children, and most of the time we have had really successful plane trips. I also tend to pack a little backpack of new things, like little notebooks or coloring books or stickers, things that my kids have never seen before. It's fun.
She'll pick up on your stress, so be positive, and know that whatever comes up, you'll be able to handle it!
Good luck!
H.