Unless you have an extreme love for Finding Nemo, don't bother riding the ride. We waited in line for about an hour for a ride that was definitely not worth it to us.
If you want to eat at sit down restaurants, make sure you make a reservation far in advance. Reservations fill up pretty quickly, especially for popular places like Cinderella's Castle.
Wear sunscreen, but don't bother bringing ponchos. They sell them there, and they're just extra weight if you don't end up needing them.
Photopass. Get them to take your picture even if you don't think you'll buy them. You might change your mind, and they are really good, quality pictures.
Learn when the extra magic hours for your trip are, and take advantage of them. They are well, well, well worth it, and they don't cost anything if you're staying onsite.
Disney's transportation is great, too. Depending on where you're staying, there is a plethora of means of transportation.
If you buy something at a gift store, you can have it shipped back to your room. It's free, and great. Because really, who wants to tote around that huge Mickey? And how much does it suck to get back to your room and find your Rock'n'Roller Coaster picture smashed?
Nobody.