As a number of other posters have said, the Park Hopper passes are a great idea. If one park is too crowed, or is just not what you are in the mood for, you can leave and head to another park. You can use the monorail to travel between the Magic Kingdom and Epcot, and I like doing so. The monorail is an attraction at Disneyland in California, and with good reason.
Also, as a number of other posters have said, be prepared for a lot of walking. Wear very comfortable shoes. The parks are huge, Epcot especially. If your feet are tired, you can use the trains in the Magic Kingdom to go around the perimeter of the park, so you don’t have to walk the whole way. In Epcot, you can use the “Friend ships” across the lagoon to the far side of the World Show case.
You can also use these boats to get to the Disney Hollywood Studios, formerly Disney-MGM Studios park. You do have to do this from the “International Gateway” entrance/exit to Epcot. It’s a nice way to travel, and is very restful.
If you get tired, find one of the many shows and rest for a while. Being exhausted is no fun. Also be sure to drink lots of water. There are water fountains all over, but the water has a lot of sulfur in it and doesn’t taste very good.
Bottled water or other drinks are probably the way to go, even though they are expensive.
Each park has a station with a board that gives you information about approximate wait times. Check these frequently, because they are updated frequently, and are really helpful. When I don’t feel like waiting on lines I will often just go to where the the shortest line is. There aren’t any BAD attractions in any of the Disney Parks, and some of them are pleasant surprises. When you go through the turnstiles in to the park you’ll get a guide to what shows start when.
Pay attention to the music, both in the attractions and everywhere else. Most of it is great, fun music.
Sit down, table service dining at Disney World is surprisingly good. Reservations are a good idea. If you look a trip planning book and think there is a particular place you want to dine, call ahead, now, and make the reservation. Avoiding common dining times is a great idea, i.e. eating early or late. The number for dining reservations is 407-WDW-DINE (939-3463).
Boma, at the Animal Kingdom lodge, is great if you are in the mood for slightly exotic food. It is the only place that I personally have tried African food, and I likes it. It’s a buffet, and does have “safe” choices for those who may not like the more exotic stuff.
You can get alcoholic beverages in all the parks except the Magic Kingdom.
The water parks are great fun and are cheaper than the other parks. If you want thrill rides there are a lot of water slides that are just as exciting as a roller coaster. If you want more mellow entertainment, the lazy rivers and wave pools are great.
Oh, if you don’t NEED a bag, don’t bring one. All of the parks have a security check point where they go through bags. These lines can be long and although they frequently move fast, if you can avoid them do so.