If it is going to be humid around the time of the wedding, you are probably best off dressing in light fabric. Choosing fabrics such as linen and cotton can help you quite a bit because they will allow your skin to breath and keep you cool. You should probably try to stay away from silk due to the fact that you could get a little heated and maybe even get some sweat stains on your clothing.
Women will usually wear a great sundress or a nice skirt and dress shirt ensemble. Sandals are perfectly fine if you are attending a less formal event, with pumps fitting the bill for an evening or more formal gathering. Men can pull off a great look with dress khakis and a nice cotton shirt and loafers. If it is more of a formal event, men are best off to wear a great suit complete with a tie.
If you are attending an outdoor event, you are going to want to make sure that you are wearing some sort of sun clock. Don't forget the sunglasses and your dancing shoes and you should be in for a great summer wedding event.