I was second photographer at a friends wedding at the weekend, my girlfriend was their bridesmaid and whilst they had a professional he was only covering from the car arriving to getting to the reception, I did the preparation and reception stuff.
Things I learnt which might help if you do end up doing it;
1) Don't underestimate how dark the venue is, in the reception I was down to 1/60th at f2.5, ISO 1600!
2) Know your kit inside out, I was hoping my 5D Mkiii would have been there, looking back I was much better off with the kit I knew well rather than learning a new body on the job. Make sure you know how to change settings on the fly without taking your eye off, despite what you would imagine things are quite fast paced when you're trying your best to record them.
3) Take backups of everything, I was using two bodies, a 1Ds Mkii with 24-105 and a 1D Mkii with 135mm F2, each with a flash. I shot loads of cards, keep changing cards when there was a lull so if one corrupted I wouldn't have lost the whole day, I also had a 32Gb SD card in each camera's second slot as a further backup. I kept all the cards on me in hard cases, then as soon as I got home I downloaded them, backed up my computer to an external hard drive and kept the full memory cards offsite in case of catastrophic fire or flood.
4) If you're using heavy gear make sure you've got a way of carrying it, I used a Blackrapid DR1 dual harness which was perfect, if I'd been relying on the standard neck straps I think I'd have done myself an injury!
5) Keep thinking of new approaches, get up high, down low, in close, out far, to make sure you've got a wide variety of shots, and take a few of each view to allow for folk blinking.
Not an exhaustive list, just a few tips that might help a fellow wedding beginner.
