1. Get to know the Bride & Groom first & find out what they want - organise a brief with them and follow it but also add much more. Also try and get to know key guests such as the parents and best man, by name. 
2. Always smile and be happy all day. No exceptions. Be positive all day, even if its raining. Never dwell on the weather.3. Be accommodating to everyone from the Bride & Groom to all the guests.
4. Don't treat the day like a Photoshoot (unless that's what the couple want) - its a wedding and their big day. You should be able to capture everything inconspicuously (apart from set up shots).
5. Get familiarised with the venues. Introduce yourself to the Vicar/Priest and find out what you can and can't do. Find out about parking and make sure you arrive in plenty of time! Never be late. Plan to arrive at least half an hour before you're due to be there.
6. Ask for a list of guests required in the group shots. If a list isn't forthcoming, follow the regular pattern. Do this as quickly as you can to minimise disruption. No-one wants to hang around for ages getting thirsty. Shout and organise people with a sense of authority but maintain that smile/humour. Never get annoyed if guests don't comply.
7. Do not drink Alcohol, even if offered.
8. Take advantage of the light - Overcast days are perfect (don't tell the bride and groom) and harsh sunlight has its benefits too. Don't shoot groups in direct sunlight, find shade. It very rarely rains all day so use the break in the weather. Sneak out with the Bride and Groom just before dusk for 5 minutes (if they want).
9. Shoot with remote flash and double up with natural light. Try not to use direct flash. Watch out for blinking!
10. Back up! - Spare batteries, spare camera, lenses, memory cards, everything fully charged up. Take your chargers too. Afterwards back up the images on 2 separate hard drives.