Mikesmom said it well. We've done 5 NYE's so far, all at Epcot, our favorite place.
Forget about doing rides. For example Test Track gets a 2.5 to 3 hour line. If you can get some fast passes that better, but those all go early, usually all gone before noon, and even the FP lines are much slower than usual because of the crowds.
Get your PS well ahead of time. Even with them you'll probably have a fair wait. We've gone to get our PS seats a couple of times and have seen people who just walked up and were totally surprised they couldn'tget in at all. Couldn't even make a preferred seating for even laterin the day. Just like FP, all the PS can get used up.
Even the snack bars will get long lines.
The parks do have a graduated closing method. They first close to off-site walk-in guests. On-site guests are the last group to get shut out so you are in better luck there. I've seen it vary from year to year, sometimes guests can get in at later times, sometimes not.
It's a toss up about using Disney transportation or your own car. Both will be slow. If you drive, you may get backed up at the entrance booths. Busses can bypass, but I've also seen it where there were so many cars even the busses were stuck in traffic. Again this varies considerably from year to year.
Enjoy the evening and treat it as a totally different experience than you would for a regular park visit. You're going for NYE, not to be going to a park just like any other day.
And mind the CM's who will be doing their best to control foot traffic. One year we were there, in front of Mexico, there was actually gridlock and nobody could move. If everybody stays to the right so there's two way traffic, it works much better.
In the past, at Epcot, there have been three fireworks displays, They have illuminations at 9:00, so that families with kids can leave (but don't expect to notice any big difference in the crowds.) Then about 11:30 then have Illuminations again, then a short recess, followed by the NYE fireworks at midnight.
Be sure you've picked a spot to watch fireworks. People will stake out the best spots in the afternoon. If you've seen Illuminations then take your time and stand back a ways and you'll be OK even if you wait until shortly before everything begins. In the past, the NYE midnight fireworks have all been high in the sky, so you don't need to be at water's edge.
Don't try to move after the lights go out. With the crowds that big and poor lighting you'd have a tough time moving about.
If you're not tired, stroll around until 1:00 am and let a lot of the crowd get out. That'll make it easier to leave yourself. (For a couple of the NYE stays, we stayed at BWV or BCV so we could just walk out. That's was really nice)
If it pours down rain, there won't be hardly any crowds left at all by midnight. That happened to us a couple of years ago.