If there's one thing you commonly hear about weddings besides the bride being beautiful and it being a nice service it's that the sound was terrible. Many quaint little churches have inadequate sound systems and there's not much the average couple is going to be able to do about that. The reception however is another story entirely.
By and large live sound guys are not DJs but I'm pretty sure we've all done a wedding or two for a friend or a family member. When ever I do this I'm on a mission to make all the mobile DJs in the area look like fools. Most of them do a pretty good job of this on their own but I like to set up an example for comparison. I should stop here and say that a good friend of mine is one of the best DJs in my area. He uses good equipment, is conscious of volume levels, and knows when to be Mr Personality and when to just play some music and stay out of the way. I don't know how many wedding receptions I've been to where the DJ thinks he's rockin the club scene. It's too loud, he won't shut up, he hasn't EQ'd his mic or the system, and doesn't gauge the crowd to see if they want to dance or if they're trying to talk.
That said, here's the simple process for getting it right when you take care of that special day for a friend or relative. Start by getting there early and EQing for the room. Then EQ your mic so people will be able to understand you. Also, shoot for max gain before feedback and throw a compressor on there. You might know how to use a mic but the drunk maid of honor is going to hold the mic by her belly button while trying not to cry and talk about high school memories. Be prepared for that.
If it's a big room and you've got the technology, set up delay speakers instead of trying to cover the whole room from one spot. And no excuses for this, I've done it with nothing more than a powered mixer with built in effects. It can be done.
Then all that's left is to figure out the music and be prepared to watch the crowd. Typically you'll be working off the bride's iPod so ask her to set up several play lists. One for pre dinner that's up beat but not over the top. One for dinner that's quiet, like light jass or classical piano. Even if it's a heavy metal wedding you should keep it light during dinner so people can talk. Then have some dance stuff that the older crowd will be in to. Think of it as a walk down the pop music hall of fame and take care of each decade. That way Aunt Edna can shake her bon bon for a minute and then leave before bed time. If you brought subs you can turn them on now but hold back the volume. Once the older generation starts to head for their cars, then you can turn the subs up and play the stuff that the bride and groom really love. Keep the subs aimed at the dance floor though because while the oldest people will leave, many more will stay and they're not necessarily up for line dancing or a dub step marathon. You can keep both groups happy with some careful placement.
Now for the mic. Just stay off it. Even if you've won awards for your public speaking it's not your day and nobody wants to hear you. It's your duty to do a good job announcing what needs to be announced and then stay out of the way. Before it's time to introduce the wedding party, grab a card and meet them all face to face if you can and learn how their names are pronounced. Then write them down phonetically in the order of appearance. You won't get applause for getting it right but you'll look like a schmuck if you get the mother of the bride's name wrong.
Apart from that and announcing the first dances you should pretty much just keep that wireless mic shut off and in your pocket. Be ready for the toasts and to make announcements about lost children or the bar closing and that's it. It's not your day, you don't need to say anything.
So take it from there Brethren of the Knob and Fader. When you offer your services for a friend's weddinig (or are drafted) just do like I know you always do. Be invisible and provide the cleanest, clearest, most tasteful sound you can. Just make sure you practice hiding your distaste for Lady Gaga songs in advance so you always look smooth at the reception.