Be realistic in your expectations! We were in Epcot last year on New Year's Day and the wait for soarin' was 280 minutes, yes, it was 4 hours and 40 minutes and people were still getting in line and plenty of others were complaining about the line and fastpasses being gone already.
We have been in Disney for Christmas & New Years the past 6 years and we usually stay at Yacht Club (walk into Epcot/DHS) or Polynesian (MK or monorail) and we go in early and leave when we are tired/start to lose the holiday spirit. I can't stress enough about the be realistic part - you can't expect to get into the MK on Christmas Day if you don't get up until noon, it will be closed to people entering by then. If there is something you really want to do, get in the park and do it. Use fastpass & EMH where you can!
I agree with making ADRs. Even if the restaurant you really want is not available now, make a ressie somewhere else so you have somewhere to eat. It might save your day if you have somewhere you can go and relax even if it was not your first choice.
My last tip - be prepared for any weather. We have been there when it rained almost every day all day, it has been cold and we were glad we had our winter coats, gloves and hats and then we have had trips where we wore shorts all day and most nights.