No naps before the trip it makes it easer for her to sleep during the trip.
bring warm drinks such as warmed milk or coco and some juices,
bring the child's fav blanket and other familiar common everyday things to help make her feel secure and at home,
bring some children's books and things that you can interact with the child with as a distraction from the constant changes,
if you have waited blankets and cloths now is the time to use them, the child will not likely want to remain seeted for long durations and so these things may help.
bring uncommon and unusual treats and foods you know the child will enjoy, keep her full of mostly things made of fiber, no sweets, use dried fruit and nut snacks or breads, it will help her sleep.
when traveling try at first placing the child in a place of good view, who knows, the child may actually like knowing whats happening and seeing things as a distraction.
careful of the toys you may bring, others may not be as appreciative of the sounds they make during the long trip.
earphones and music are a good thing to help dull the sounds around her as well, keep it playing when and as she sleeps and in this way many sudden sounds others make may not bother her as much.
when ever you get a chance to stop, let the child run a little so she can see and feel the new things and changes, this could help her feel more secure, remember life is so new to a child that age, most everything is always new, and this also will help her stretch her legs.
and remember, even if the child is potty trained she will hardly be able to go off on her own or the bathroom may always seem in use, so bring plenty of diapers and wipes and a bag to put the soiled ones in.