The Walt Disney World parks each stand a short drive away from the others. While you can easily reach them on the monorail or by car, it can be a bit of a chore to travel from one park to another. Accordingly, you should set aside one full day for each park you want to see, rather than jumping from one to the next. Stay away from the Park Hopper add-on to your tickets. (It lets you travel to more than one park on the same day.) If you don't have the time to visit all of the parks, decide which ones you want to see. Magic Kingdom makes an ideal first choice---it's Disney World's original park---followed by Hollywood Studios and/or Animal Kingdom. Epcot is nice if your party consists of adults or if you need a quieter day in between more hectic parks.
Peak season at Walk Disney World takes place during the summer, the two weeks around Christmas and spring break (mid-March through April). If you go to the resort then, expect a lot of crowds and long waits in line. The first few months of the year (January through mid-March) and September through November are often much quieter, and while you won't see as many fireworks displays or similar events, you're apt to see much more of the parks themselves.
Sun and humidity are constants in Florida, and Disney World visitors should prepare for both. Take sunscreen with you and make sure everyone has a broad-brimmed hat. Because rain is a possibility, you may also want to bring a poncho or a portable umbrella with you. (Disney World will cheerfully sell these things to you, but they charge an arm and a leg for them.) Similarly, every Disney World park involves a lot of walking; be sure to wear comfortable shoes every day and take a stroller if you have small children.
Most Disney World rides entail a long wait in line; it pays to bring distractions like coloring books or handheld video games, especially if you have kids. Disney World offers Fast Passes for many of its most popular rides---enabling you to come back at a later time in exchange for a short wait---and you should make use of them any time the park is crowded. In addition, pay attention to rides with a minimum height requirement; you won't be able to bring small kids on them. However, Disney allows families with small children to "swap" kids at the end of the line---letting one parent go on the ride while the other one waits, then letting the other parent go on the ride. It lets you stay together while still enjoying all of the rides.
