Cheap, disposable ponchos in case of downpour. (You can toss them away afterward.) 79 cents at
Walmart, Target or K-mart (camping section).
Bodyglide and moleskin for feet. Take care of your feet! It can ruin your trip if you have to deal with blisters.
Start "training" for all the walking you're going to do. You can log between five and ten miles a day at some parks, depending on how long you tour. Work up to walking a few miles, several times a week, for a couple of months prior to your trip.
Get there before the park opens, and leave (take a break) when it gets crowded. You can get a lot done in the first few hours of the morning, before it gets really crowded.
Avoid Extra Magic Hour (EMH). Some people love it, but I hate the crowds! One strategy is to see what parks have EMH, then go to that park the
next day! Because so many people have done EMH at that park on the EMH day, they go somewhere else the next day.
Use cell phones to communicate if you can. It saves lots of time and hassle if you want to split up. Rather than saying, "I'll meet you ___ at 11:00" you can communicate and change plans if needed.
Use fastpass as much as you can. (See these boards for info on how it works, if you're not familiar with it.)
Make a few ADRs (dining reservations) well in advance, especially if you're traveling during a "
free dining" time. You can always cancel your ADR if you change your mind or want to be flexible, but you might not be able to get reservations at the restaurants you want, at the times you want, otherwise. You can make your reservations months in advance (180 days before your trip).
Get the Unofficial Guide to WDW. It's full of great advice about hotels, restaurants, touring plans, etc.
Most importantly, as another poster said, scour these boards for information. I have found them so helpful. They're a great source of information. Once you have some ideas about what questions to ask, you can then ask the "experts!" Often, the more specific the question, the better.