1) Do not wear white! This is the biggest "no no". The Bride is the only one who should be wearing white, and should stand out the most. A bit is okay, such as an accent in a dress, but not solid white or cream.
2) Do not wear anything super bright or neon. Colour is good, but you don't need to wear lime green and take attention away from the newly weds.
3) Keep makeup neutral. You want your makeup to look fabulous, but keep it simple, this way you will look classic photos that will last a lifetime.
4) If it's sunny/warm out, wear your hair up. it will be out of your face and off of your back, and will help to keep you cooler.
5) Pack blotting papers in your purse. If you get hot and sweaty, blotting papers will help absorb sweat and oil from your face, leaving your skin shine free and photo ready.
6) Be prepared! Pack some tissues in your purse in case the day gets emotional. Bring some bandaids, you're heels might not be as comfortable as you thought.