UPDATE November 2011: I got these sweater bags from Dollar Tree.
$1 for 2 bags. They work perfect for this.
Ever since I was 16 I always had a "just in case" box in my car.
Now that I have children that box has grown and grown and grown.
I have passed out tips for an "on the road family" before and now I have another one.
Maybe it is just my children but there is something about cuddling
up with a blanket that just helps them stay calm in the backseat.
I use an old bedding bag to keep all of the blankets in. I fold them tight and pop a dryer sheet in there with them so that they always smell fresh when we pull them out.
Ours stay in the hide away compartment in the back of the car until we need them.
These also come in handy when the kids are sick or wet, if we need to protect the seats. ;)