Good tips
It's always the simple things that get overlooked, and certainly having some kind of "gameplan" is the way to go. You could be the be the most skilled artist behind the camera (and infront of a PC), but if you're in the wrong place in the wrong time, you've simply got nothing to film!
I suppose the most important tip would be to film a rehearsal - if anything, just to ensure you get the best vantage point with nothing obsuring the view. Or establish the best place to leave a static camera - something you can keep rolling for cutaways.
Ensuring people are HEARD is always my downfall. Its amazing how poor built in mics can be, so investing in an external mic would give a good return.
The last wedding I went to had disposable cameras placed on all the tables at the reception - a good way of ensuring a whole range of photos. It wouldn't be possible to do this with video, but has anyone ever passed around a camcorder? Some people can feel put off by a camcorder thrust in their face, but they may warm to the idea of picking up a camcorder that;s been [assed to them. May get a few rough diamonds that way LOL - but probably an idea best consigned to the "bad idea" category.
Well, I think I'll make this a sticky to see if we can get some more ideas