A couple of years ago, I was at Disney for New Years. The best advice I can give you is to make as many plans as you can in advance, including ALL of your restaurant reservations (if you plan on making any). They generally allow you to book those 3 months out, and you might seriously find that EVERY restaurant is already booked this close to Christmas. Crazy, I know.
Also, plan on getting to the parks every day as soon as they open. On holidays, the parks tend to reach their capacity limits and sometimes they are forced to shut them down. This happened to us on NYE. They closed the Magic Kingdom around noon. If you were in the park, better not leave or you won't get back in. The same thing happened with Epcot closer to 4pm. So be prepared if you plan to be there for Christmas Eve or Day. Things get nutty.
If there are any other attractions you are hoping to see that take reservations, like a princess dinner, breakfast with characters, etc., book all of that now.
Other than that, just be prepared for crowds and long lines. If you can, get a parks pass that includes Fast Track, which allows you to skip ahead of certain lines. And remember to take your patience pills while you are there, you will need them : )