Here are some suggestions for preparing for this event. Most of which I'm going to do:
Plan ahead of time what you want to get done.
There's going to be a lot of people. Even though you get 24 hours, most of it is going to be spent waiting in line. So make sure you set some goals. My goals are to try out some of the new food and drinks they're releasing for the event.
Pack light but right.
Okay I'm only following half of this suggestion. I am going to be so prepared. Here are some of the things I will be bringing to the event: reusable water bottle, a cardigan, a sweater, a change of socks, an extra tank top, a hand towel, sanitizer, sun screen, plastic baggies, and most importantly....a battery pack. I can probably explain each thing but that would take too long.
If you are like me, you love water rides. Even at night. I'll be bringing extra socks because I know it is the worst thing ever to walk around with soggy socks. I will also be wearing an extra tank top in case my clothes get too soaked and I want something dry to change into. Of course, I always take off the outer layers when I get on the ride so I'll have something dry to warm me up afterwards. And because it is a 24 hour event, you know it is going to get cold in the night. Bring a sweater or jacket to tie around your waist or put in your bag for those colder nighttime hours.
Bring charging supplies!
I love taking pictures with my phone or sending pictures to my friends when I'm in the parks. Unfortunately, that means my phone battery usually runs out after about 6-8 hours. I, as an always prepared person, always carry a charging cord, adapter, and an extra battery pack. I won't tell you all of the places in the park that you can find an outlet to charge, but there's a few inside Flo's Cafe and some inside the Main Street Theatre.
Keep hydrated and well fed.
This is a thing we often have to remind ourselves when we're having fun. It is very important to drink a lot of water while you're at the parks. Being out in the sun and walking around all day can dehydrate you. I recommend bringing a reusable water bottle and going into any of the restaurants with soda fountains and filling up there. (Or you could go up to a quick service counter and ask for water. They will give you a cup of ice water. But I don't recommend this because it wastes cups.) And of course all that fun and games will leave you famished. I suggest light snacking throughout the day and/or a few meals to help keep your energy up in order to survive the night.
I know that this is a given but you should definitely sleep well the night before the event. And maybe find a place to nap during the event. Popular places include the Animation Building at DCA and the Main Street Theatre at DLR. Take caffeine if you must (I personally won't).