Make a list of everything you will need to bring with you on the big day. Gather everything in advance and put it in one spot so that you don't have to search for an essential item at the last minute.
If the weather will be very hot, make sure you will be in air-conditioning for as much as possible so that your makeup does not melt!
Go through the timeline of the day carefully and make sure there is someone who is managing it and telling people where to go or what to do next.