Get there early (before the official park opening of 8:30 am)......you will get an earlier dolphin swim time, which is particularly good if the weather gets kind of wonky in the afternoon, as it is known to do in Florida.
Come hungry.....breakfast is provided, as well as lunch, snacks, and drinks (including alcohol).
If all drivers in your party wish to partake in the unlimited free alcohol, take a shuttle or hire a taxi so you don't need to worry about driving.
Plan to spend the WHOLE day. You will be amazed how quickly time will pass, and you will be wishing you could stay longer than park closing.
Budget for a photo package, especially if you are doing the dolphin swim. The pics are great and a wonderful reminder of your day.
Buy your tickets early. Costs only go up as tickets sell and availability for your date decreases.
Use your free admission to SeaWorld and Aquatica. Both are well worth a visit, and make your DC admission cost much more reasonable.