OK, so you're not really talking about a Christmas trip as much as a Chrismas WEEK trip - and that's a whole 'nother thing!
The first time we went to WDW, we went Christmas week, and this week really hammers home the adage 'failing to plan is planning to fail'. My strong advice to you (and what we did) is to search the forums for New Years Eve (or NYE), and start reading and taking notes. Short version of my advice is:
1. Get to the parks at opening. Or before. Definately before. Sunrise good

!
2. Ride the rides you want to right away. Get Fastpasses, but hold them until the afternoon, when the lines for rides will be hours long and all the Fastpasses will have been given out. You can ride later than the time on the pass, but not earlier. And you can hold more than one at a time (the passes will have a time on them when you can get another pass).
3. Make ADR's for lunch AND dinner. Even if you'd usually eat at CS. Trust me. You'll thank me when you see other people waiting in line for an hour and a half for a burger.
4. Realize that the parks will get more crowded as the day wears on. So go commando on rides in the morning, then spend the afternoon and evening enjoying the parks, seeing shows and parades, and people watching.
The parks are VERY crowded, but you'll have fun!
KC