Comfortable clothes: especially for the getting ready part of the day. This is when many brides have personalised gowns, and comfy joggers and tops.
Water. Absolutely ensure to drink lots of water, and bottled water, although not eco friendly, is imperative to have around. Ideally you could have reusable bottles...maybe personalised :)
Mouthwash or toothpaste. Carry a small travel toothpaste and toothbrush, or mouthwash, just to do a final swish before you leave for the aisle.
Mobile phone charger. So important in today's day and age to have this handy as you'll want to take images!
Tissues. You will definitely need these as weddings are an emotional affair.
Blotting papers. Not nice but of course even brides sweat on their wedding day and you'll want to keep your makeup top.
Deodorant: spray and roll on. Keep applying it to keep fresh.
Safety pins: probably the most important part to have. Works for dress mishaps and holding things together. As a photographer I always take these, among other things, to every wedding. Only once thankfully did I have to pin a hole in a bridesmaids dress.
Travel Sewing Kit: as essential as safety pins for pretty much the same reason.
Pain relief: weddings can be a very long day for everyone, particularly for the bride who generally doesn't get much sleep the night before and is up early to get hair and makeup done. Many weddings won't finish until gone 11pm too!
As a wedding photographer I aim to bring with a little bag of all these things but you can also ask your Maid of Honour or Bridesmaids to carry this in their bags.