Know Where You Can Hang Out Indoors
If for some reason you didn't pack enough layers, you may occasionally need a place where you can hang out indoors a while and warm back up. Gift shops are ideal for this, especially in Magic Kingdom—using the shops there, you can go from one end of Main Street, U.S.A. to the other without ever stepping foot outside.
Of course, there are also plenty of restaurants and attractions at each park that happen to be indoors, so do some research beforehand and you'll always have an idea of where you can step inside when the weather gets to be too cold for your tastes.
Wear Sunscreen (Yes, Even in Winter)
Cooler weather doesn't mean you have to stop taking care of your skin. There's still a lot of direct sunlight happening on any given day, so you'll want to be prepared by bringing sunscreen with an SPF rating of 50 or higher.
If you're going to be in Disney World just before Christmas, why not stop in at one of their Christmas-oriented gift shops—such as Ye Olde Christmas Shoppe in Liberty Square (Magic Kingdom), or the larger Disney's Days of Christmas marketplace in Disney Springs—and have a personalized, hand-painted tree ornament made?
There you have it! 9 tips for visiting WDW in winter. Hopefully this list gives you a great starting point to build off of.