I live in the South, and know all about what August is like! We are traveling to WDW this summer with family from California, and we have been trying to prepare them! August is an extremely hot month, but coming from the west coast, I find the humidity to be the worst part of it. As the previous poster mentioned, it is very important to take advantage of the coolest times of day. In Atlanta, we do everything outdoors in the mornings and then go indoors until the late afternoons/early evenings. Plan on heading back to your hotel for a pool/rest break around lunchtime, and heading out again later in the day. The nights can be my favorite time, warm but not so oppressively hot! It is really important that everyone stay well hydrated. Limit caffeinated drinks, and drink lots of water. Go to an indoor ride or attraction every so often to take advantage of the air conditioning. I sometimes take an icy cold shower in the afternoon just to cool down!
There can be a lot of rain in August. Oftentimes, they are just brief showers, but make sure to pack ponchos, so you don't get caught off guard. Often the rain strikes on the sunniest days, so just always be prepared. The rule at our local pools is that everyone is out of the water the minute you hear thunder, so I think that is a good general safety rule.
Summer is also mosquito season, so you may want to pack a sunscreen that has repellant built in. I also plan on packing some Caladryl to relieve the itching from the bites!
On a more lighthearted note, the humidity wreaks havoc on hairstyles, so you might bring a de-frizz conditioner or spray, if that matters to you! The good news is that it really moisturizes the skin!