As above + reccie the venues first if you can. Check out the minister`s/registrar`s attitude towards use of flash and even amount of shooting you can do without flash - one registrar for me said he did not want to hear incessant clkicking and limit it to 5/6 shots during service ( I was a little naughtier as his assistant told me he was a little hard of hearing).
Back up camera and plenty of spare batteries for flash and camera. It will be a long day - make sure they lay on a snack and you have plebty of water. set up the formals,ask those jostling you with cameras to take quick shots and leave as you are the one they are paying - then re-compose and shoot - at least 2 of everything for blinkers. If it is a token payment rather than what a photographer would cost manage their ecpectations by being up front with what you can deliver. For portraits shoot with a large aperture - make your shots look different from the geust shots.
Good luck
