Have something small for the child to do/play with when waiting in line. Ive done stickers, coloring, bubbles, snacks etc while waiting in line. I always bring extra for those around us and we get to entertain a lot of peoples little ones
Water..water...water.. did I mention water??? Make sure your child is drinking plenty of water. I know mine would get so distracted they wouldn't want to drink. I made sure to encourage water anytime we went back to the stroller. The more they were walking the more I encouraged them to drink.
Pack snacks.. I know mine have issues sitting for a full meal when they get excited so I always kept healthy snacks in my bag for the sudden "mommy Im STARVING" which usually happened about 20 minutes after the lunch they didn't eat.
Follow their lead.. Don't get so intent on seeing what you want to see that you ignore what they are trying to see. Sometimes the most awesome exchanges are when your child sees something in the decorations that you have been to busy or distracted to see.
Book character meals later in your vacations and try out the characters before hand if your child hasn't had much/any exposure to them. Some super shy children bloom around characters and outgoing children will hide. Plan the meals to give you the opportunity to cancel them if need be
Know your kiddos. Mine will not rest at a hotel, if I take them back to the hotel then it was a sure way to ensure hyper children and then grumpy ones later on. If we just let them lay in the stroller they will either rest or take a nap which makes for happy kids later on
Consider pull ups for at night even if your child has been potty trained for a while. Exhausted young children have accidents even when they have been accident free for years (happened to my 6yo niece much to her great embarrassment

)..
Costumes are cute but not all kiddos find them comfortable. I like to pack an extra outfit for days the girls wear costumes so if they start getting itchy they can change.
Disney bandaids solve all ouches.. Seriously, Ive have benefited so much from a box of mickey bandaids in my bag, again I carry extra and offer them to people if I see there is a need.
Bring LOADS of patience. You are going to need it. Nothing is sadder than seeing a parent losing their cool with their little one at Disney. Bring your patience and if you feel yourself losing it take a break and cool down some. A cup of cold water and sitting for 10-15 minutes usually makes me feel a whole lot more human.
Determine an amount for souvenirs and then put it on a gift card. I always save shopping for the last day and the kiddos can only spend whats on their card. I don't add or help with a purchase. If they can't afford it then they don't get it. Yes, even my little little ones I do the same thing.