There are friends who want to serve as singers as well. And though of course, we want to give each singer-friend his/her chance, we do need to limit to around 2 to 3 songs, unless they are willing to sing while everyone else is eating. :)
Most notable are the singer-friends who really have great voices. They add value to the program and cause people to stay because people want to hear those who can truly sing better than they do. Last weekend, the couple chose to have their friend sing and I must commend them... their friend really has a golden voice. Last year, I had a little boy who performed for everyone. No one went home while he was singing, rather people stood up to watch closely. That's how awesome he was!
Needless to say...
It's also important for these friends to choose songs which aren't just couple favorites, rather songs that people can relate to. That means, they should go for classics and avoid tunes which border on rock. Why? Because when the ballroom/pavilion/reception area becomes too noisy, people leave -- after all, they've eaten already. They don't need to stay and have their eardrums tortured.
Choose those who are close to you, but give them ample time to prepare. Don't surprise them on the day when the emcee calls for them. Tell them that you need them to speak during the reception and ask them to prepare but to kindly avoid reading the whole speech. They can hold on to the paper but try to speak without looking at it. (the same way you are advised to avoid reading your vows so you can look your partner in the eye)
Tell friends to keep inside jokes to a minimum since they aren't speaking only to you, but to the whole audience. The audience should be able to understand what they are saying so that the audience will listen.
A good rule of thumb is to keep speeches within the 1 to 2-minute mark. That way, the speaker leaves the audience wanting more, so to speak.
They can add value to the program if they are chosen well. They should be the type who are willing to play -- not too shy, please. :) Also, choose a game which your guests can easily understand. That way, they are willing to watch and listen. Think of your space constraints as well. If you have very little space, use this as a limiting factor when choosing from your list of possible games.
Bride and Groom:
a) Remember your entrance details. You can forget everything else -- the speakers' names, the singers' names, etc. But... DO... NOT... FORGET... your entrance details. If you are supposed to enter singing, don't enter and smile and wave at everyone and forget to start singing. If you are supposed to open the door for the bride, because she should enter at a certain point, don't forget her either. (And if the groom/bride tends to be forgetful, advise the coord, maybe they can make signs and raise them up so the groom/bride would be reminded to go and pick up your partner now!
b) If you can prepare a choreographed dance, even if you just copied the steps in youtube, go for it. This is not just for your video. It's also for the guests to feel how well you prepared for the program. They will be willing to stay because they will think that there must be more up your sleeve and they should stick around to find out.
c) Prepare your speeches -- the welcome speech as well as the farewell speech. We are asking everyone else to prepare, so definitely, you, being the stars of the show, are not exempt from preparing.
d) During the pictorial and buffet process, always remember that there are other table occupants waiting for you to finish your pictorials with the current table's occupants. Thus, keep the chitchat to a minimum. Otherwise, you will cause some to just stand and go straight to the buffet without going to your area for pictorials.
e) Prepare your prizes. You've asked guests to prepare by dressing formally. You made them go to church and wait for your march. You asked them to wait again at the reception because you will have your pictorials as a couple. You should at least make their participation worth their while by giving prizes that they can appreciate and use. Avoid having games without prizes... please... :)