Cant say this enough, having experienced wdw as a cm and a guest. Go early and stay, especially if you want to be in MK on NYE or Xmas Day. Traditionally MK is filled by 10am and the other parks follow around 11am.
If you leave your chances are good that you will NOT be able to return. If you have a handstamp or a ps in the park you are NOT guaranteed entry.
IF you are a Disney resort guest you are guaranteed entry into a park, but it may not be the park of your choosing. Upon check in at a Disney resort during this time period all guest are given a letter stating this policy and a warning regarding crowded parks.
Wish I could offer an easier solution, but ultimately this becomes a safety issue for wdw.