My top tips for a first time visit would be...
1. Comfortable shoes - it has been said that the average person walks 26 miles during a day in the parks and I can believe it.
2. Planning - I am not a militant planner as many people are, but it definitely pays to do some planning. As others have said, you won't do it all, so make a plan of things you absolutely don't want to miss.
3. Dining - If you have the
DDP, take advantage of making dinner reservations. Popular restaurants can get booked out 6 months out!
4. Bag - Buy one of those swimming bags to take to the parks. They are light, easy to store and waterproof.
5. Rest Days - Plan to take regular rest days. It can be exhausting walking all day in the heat. You can still do other things (Disney Quest, Old Town Kissimmee, Water Park etc.) but don't plan on a park visit every day.
6. DRINK! - Make sure you drink plenty, it's easy to forget and get really dehydrated in all the excitement!
7. "Secrets" - There are a lot of secret little things you can do that a lot of people don't know about. Most of the information is freely available online. For example, the paint brush hunt on Tom Sawyer Island.