When it comes to hot weather you need to think about what make up you are going to use. Avoid liquid products as they might run, try a powder product instead. Also use a setting spray to keep your make up in place. Make sure your make up also has a spf in it. Another obvious tip is to put your hair up, this will keep the back of your neck nice and cool.
If you are organizing a wedding and you want it to be outside have you got a shaded area for your guests. Wedding ceremonies tend to be between 12-2, the hottest period of the day. Are you shaded by trees or a marquee? Or could you hand out parasols for your guests?
If you have a summer wedding and it could be very hot why not choose a dress that is a light fabric and a pale colour.