One thing that I never do is to guarantee that I will get pictures of everyone at the wedding. Its just not possible. If I'm desperately running around taking hundreds of shots of guests, just to make sure I don't miss anybody, then I'm not doing what I'm supposed to be doing - observing and waiting for the real photogenic moments.
I do my best to make a mental note of the main players on the day - parents, brothers and sisters, best man, bridesmaids etc. - and I try to make sure I've got a good selection of pictures which include these people. That's obvious, and I wouldn't be doing my job properly if I didn't.
What you need to remember is that nowadays, with probably 10-20 keen photographers at every wedding, you're going to be deluged with loads of other pictures via email, twitter, facebook and the rest, of all the other guests, posing in little groups for each other. This is exactly why I don't need to be shooting those pictures and can concentrate my efforts on getting the pictures that nobody else will get, or sometimes even see.
Its about the quality of the photographs, not the quantity.