If you’re not hiring a day of coordinator (which, I think is a GREAT suggestion), be sure to put one of your bridesmaids or family members in charge of creating an emergency day-of kit for you. Typically, these kits include things like: aspirin, band-aids, blotting papers/powder, sewing kit with safety pins, mints, chapstick/lip gloss and anything else that could be important on the day of.
Martha Stewart’s wedding site features information on how to DIY your own emergency kit. You can also Google it and find many sites with information on what to include in the kit. Any of them provide a good foundation of what to include.
You can also purchase pre-maid kits, like this one featured on Amazon.com:
What types of items would you want to include in your day-of emergency kit?