We went Christmas week 2015 (Dec. 25 - Dec. 30) and, yes, it was crazy, crazy crowded. Would we do it again? Not if we could go any other week of the year. But, if Christmas week was my only chance to go, this is what I would do:
1) Pick which park/parks are your priorities and pay to spend the money to stay close to that park/parks. We stayed at BWV and it was the saving grace of our vacation because we could walk to DHS and EP. ALL Disney transportation will be immensely crowded that week (buses, boats, monorails) and traffic is horrendous, so the more you can avoid buses (in particular), the better. I'm not a fan of the monorail resorts because the monorail is so unreliable. EP is my favorite park so to be able to simply walk back and forth, was a vacation-saver.
2) If you have to go to MK (if I went this week again, I wouldn't step foot in MK), plan on half-day, RD mornings and either make a TS lunch reservation or get out of MK to eat lunch. You will not want to attempt QS in MK this week. Literally lines out the door and nowhere to sit. At one point, in MK, the only seat we could find was at the face painting area in Tomorrowland - so my DS got a nice, relaxing face painting to get out of the crowds. See the early parade. Unless you make an MK dessert party reservation, watch the fireworks from a near-by resort (or try to get a reservation at Cali Grill or Narcoosee's)
3) Make a tour reservation. We did Wild Africa Trek during this week. Yes, it's expensive ( no discounts during this time frame), but it enabled us to tour the safari area of AK with zero crowds. If you've ever wanted to do a tour in WDW (and the tour is running this week), do it.
4) Avoid DS like the plague. It's as crowded as MK.
5) Tour EP in what may feel like reverse order and start touring the countries when they open at 11 am (or simply start here at RD to ride FEA if you don't have a FP+). Once the Candlelight Processional performances start late afternoon, it is almost impossible to walk through the middle section of WS. If you tour the countries first, then go to FW in the early/late evening, FW is far less crowded than it will be in the beginning of the day. If you want to see CP, book a dining package.
6) If you can, spring for special experiences (as I sort of already mentioned above). Reservations at restaurants where you can view the fireworks, fireworks cruises, tours, dessert parties, etc. Yes, it's going to add to an already expensive trip, but it will help you avoid crowds.
7) Pick a resort where you would like to spend a lot of time if you can't handle the parks anymore.
8) Don't expect to get as much in as you probably would if you went another week.
9) Tour the deluxe resorts! We did a monorail crawl on our last trip - started at Trader Sam's, visited GF to see the gingerbread house and had dinner at Cali Grille. My DS9 still talks about how fun it was.
10) Go to a dinner show, visit the Boardwalk at night, etc. Again, find non-park events and activities that you will enjoy that will get you out of the crowds.