I am one of those that tends to over pack. I'd much rather have it and not need it than to need it and not have it.
The average highs March are typically in the mid to upper 70s & average lows are in the mid-50s. Keep in mind these are averages. The temperatures climb a little as March progresses. A couple of days prior to your trip, go to www.weather.com & check the 10-day forecast just in case there is some out of the ordinary weather pattern moving through.
March is one of those months you can have some warm days and some chilly evenings. I am cold natured so I always take along a light jacket, sweatshirts and jeans. In addition, I bring along shorts & short sleeve shirts for the warmer days. The pools at the Disney World Resort are heated so I'd pack swim clothes (including swim diapers if you have little ones). Make sure you pack comfortable shoes as you will be doing a lot of walking.
The average rainfall is a little over 3 inches in March but just in case, I always throw in some cheap disposable ponchos, rain jackets and umbrella. If you forget these and it rains, ponchos and umbrellas are available in the theme parks. I'd also recommend a stroller cover if you are bringing little ones.
If you have any advanced dining reservations, make sure you check the dress code. Most places are casual but there are some places like Victoria & Albert's and other signature restaurants where they have a more strict dress code.