The first thing is to realize that not everything is going to go exactly as planned. Sure you've hired great vendors and planned every detail, but something will not go according to plan It's inevitable.
The thing to remember is to not let something little ruin your day!
If you have a wedding coordinator, let them deal with the problem. If not, designate someone in the bridal party to handle it (not you, your spouse or your parents!) Pick someone who is level-headed and can calmly discuss the situation while you go about enjoying your day.
Another suggestion is to have an emergency kit filled with things you may need at some point in the day. (I always bring one to all of my events just in case).
What to pack in an emergency kit
Stain-removal wipes. For spills on most fabrics (sold at pharmacies and grocery stores).
Baby powder. Great for preventing shoes from squeaking as you walk and for controlling light perspiration.
Hairspray. For tress repair.
Double-stick fabric tape. Fix fallen hems or fasten bra straps that keep peeking out of your gown or your bridesmaids’ dresses (sold at fabric and craft stores).
Moleskin. Attach to shoes to prevent blisters and as padding for sore feet (sold at pharmacies).
Scissors. To cut the moleskin and the fabric tape down to size.
Superglue. Among its uses: repairing shoe heels that snap off and fingernails that break.
Breath mints. They help with dry mouth and keep your breath sweet for greeting guests — and for your first kiss as husband and wife.
Straws. Sip drinks through a straw to avoid ruining your lipstick.
Ibuprofen. A fast fix for headaches.
Mini sewing kit. Include thread in white and in the color of your bridesmaids’ dresses to repair last-minute tears.
Clear nail polish. Use it to stop pantyhose runs in their tracks.
Tissues. For tears and much more.
It's also a good idea to pack extra everything (panty hose, contact lenses, etc.) and bring some extra cash just in case.
Designate in advance someone to take your presents home. Give them a house key so they can have everything there for you when you return.
Bring back-ups of your vows, music choices, speeches, photo shot list, etc. van if you print out your vows, bring a back-up on your smartphone or leave one with a wedding party member. The same goes for all your lists and itineraries.
One of the other most important things is to be sure you eat something. This may sound silly, but I can't tell you how many times I've seen a bride or groom feel sick because they didn't eat a thing (especially if you start hitting the bubbly!).
Also be sure to have all of your vendor's cell phone numbers and make sure at least one of other person in your bridal party has the numbers as well.
You've done all the planning and the big day is here. Enjoy it. Savor every moment.