I rehearse a lot and try to be very well prepared, but I still sometimes forget the words. I didn't used to use crib sheets but now I do, mostly to deal with requests. I try to be sure that I know a song cold before I sing it in public. But, when I draw a blank I either - 1)mumble 2)hum 3) just keep playing until it comes back to me - sometimes I can make it seem like an instrumental break 4)sing the chorus/previous verse again and hope nobody notices. I've almost never just stopped and announced that I forgot the words. Sometimes somebody in the audience will realize what's happening and they almost always wink or flash an understanding smile - usually I don't think anyone notices. I look at it this way - a mistake happens and it's over in a flash. Nobody is going to remember it or dwell upon it except you, the performer. Unless you chronically don't know the songs you sing, it will be quickly forgotten by the audience as long as you just brush over it.