The southern United States are a very hot place during the summer months. At times these parts can get over a hundred degrees, and this is not a dry heat in the slightest. If you are going to be in the South during the hot part of the year, your first good fashion advice is to find yourself a good umbrella and a sweet smelling sunblock, so you do not end up catching on fire (not literally, but you might just feel like that could imminently happen). But once you get past the feeling that you could end up as a piece of charcoal, there are some great fashion tips that could look good, as well.
For one thing, dress in very light layers. First off, they will absorb sweat so you do not look like a damp sponge (even if you end up feeling like one after a strenuous day). Secondly, the layers will actually trap cooler air between them, which can actually help you to stay cooler than you would if you just wore one layer. Just make sure that the layers you wear are not going to end up trapping lots of heat.
Then, once the functional part is handled well, you get to do something during the summer that a lot of people completely avoid doing during the winter time (even in the south)- you can put on a whole lot of bright colors. Since lighter colors tend to shed heat better than darker ones do, the South looks like a massive flower garden during the hot months. After all, the locals already know that lighter colors will help them to keep cooler. From there, you just have to bear it.