Keep in mind that NYE crowds are as large as they get. We've done NYE 4-times now, (No 5 will be this year NYE Dec 31, 2003)
Do not plan on doing many attractions as the lines are terrible. We've always gone to Epcot and Test Track will typically run 2-1/2 to 3 hour lines. The fast pass tickets are usually gone for the day by about noon. There's lots to see and do, but don't plan on doing much of the regular park attractions. Save that for another trip.
Make your lunch and/or dinner reservations. All the restaurants will book full so getting in can be very tough. I don't know how many times I've been at one of the Epcot restaurants on NYE and someone walks up about 3:00 pm to ask how long the wait is. Their expressions are a Kodak Moment when told the restaurant is full for the rest of the day.
Keep in mind there are capacity limits at the parks. MK always fills first. Disney has a (somewhat) secret formula for when they shut a park down for more guests, and on-site resort guests are always the last group to get shut-out, but if the weather's nice, don't take chances. Get there early and stay.
If you're lucky, hope for forecasted rain that doesn't happen. That'll help keep the locals away.
Also, at Epcot, everything dbond mentioned is neat, but the midnight fireworks are the best.
Have a great time.