I'm sure most of it has been said in the links above and what SDK said.
The most obvious hints/tips - Take spares of everything - batteries (camera and flashgun), bodies (film and/or digital), lenses, film/memory cards(a few small capacity ones rather than one large capacity), mobile phone (in case you are held up etc) - maybe extreme but arrange for someone to be available to drive you just in case your car goes belly up - believe me it happens!!
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Make sure you know the time of the wedding and the time of the reception so you know you have time for photos. Make sure the b&g know to allow at least an hour/hour and a half for photos after the ceremony and before the reception starts. Check if you are allowed to take photos in church with/without flash.
Have a list of formal pictures the b&g want and give copies to the ushers and the best man. Make them do all the organising and rounding up of people for the photos. Last thing you need is to be running round shouting for people.
Do the big groups first and work your way down to the b&g. This way the guests can wander off to get drinks etc.
Above all, have fun! If you look like you are having fun, the guests react better to this, the b&g will relax for you!
As SDK has said, see if you can be bagman for a local professional. He/she will probably be glad of some company!
Best of luck!
Andy