If you've been playing Rock Band as much as we have, you've tried to sing and play guitar/drums at the same time at least once. Unless you've got a mic stand lying around, you had to use either a human mic stand (I tried that; she got tired and started choking me with a Wiimote) or somehow dangle the microphone from something. No more! Here's an easy way to play and sing: disconnect the microphone, then plug in the Xbox Live headset (the really cheap one that you get with everything) into a controller.
When you start up a game, the it'll will ask you to confirm that you can hear yourself through the speakers before you start. Just shout something into the headset and make sure it comes through before you confirm and start rocking.
Bonus tip: if you're playing on Xbox Live with someone else, you can actually pipe your vocal track over to their side by holding down the right bumper on the controller. If you're using a real mic (and not playing and singing), you can use a rubber band to hold down the bumper—a la the Superman Returns achievement—to make sure it's always enabled. ROCK OUT.