This probably goes without saying, but be sure to test the reach of your wireless transmitter under “worst-case scenario” conditions BEFORE you use it. Hold the microphone close to your body and walk up and down the entire stage. When dropouts occur you can usually get away with simply adjusting the antenna to a better position – when this fails, your best bet is to avoid that area. Those people known for their wild stage shows should be aware of the range of the transmitter, so that no unpleasant interruptions happen during the performance.
Also, it is prudent to do another brief test right before taking the stage. In some circumstances additional equipment that may become active at show time can step on your wireless signal. This is of particular concern in broadcast situations where there can be lots of additional radio frequencies used during a show.