You need to get to know the best man/ushers.
They need to be the ones sorting out the guests for the photos.
You'll probably find that if you shoot the shots in reverse you have more control.
Start with the group with everyone there, before people walk away and then work you way down till our left with the B+G. Stops people getting bored, and will also mean that you will only get the people desperate to get shots milling around.
Have a word with the B+G to make sure that they are looking down your lens, not everyone elses, otherwise your shots will be ruined.
Don't be afraid to whack up the ISO in the church. A grainy sharp shot is better than a blurry shot. It can at least be converted to mono and used. Blurry goes in the bin.
You should borrow the body, You can leave it in the bag, but it is there as a backup, just in case yours stops working. But have a play with it at home so you have an idea of how to use it.
Use the flash on ETTL, even outside. Nowt wrong with a little fill flash.
If you can get it off camera, great, if not then try to bounce it if your inside.
Have a list of the shots the B+G want and make sure you get them.
Don't bother shooting during the meal, people are not attractive when eating.
Have something to eat and drink yourself. You'll be knackered. Have a Red Bull and a Mars bar in the bag.
Make sure your batteries are charged, get an early night.
Work out a system with your memory cards - you do not want to be sticking a card in that is full and wasting seconds, and you don't want to lose them. Will you have time to back any shots up to a laptop during the day? Sooner they are backed up the better.
Have fun and good luck.
