I have to sing it over and over and over again before I'm confident of being able to do it in a performance from memory. Writing it out from time to time definitely helps with getting it into the brain. However, for the purposes of actually performing I find that the safest path is to think ahead whenever I have a break - between lines, I'm considering what the next line's going to be; between verses I'm trying to recall the structure of the following verse or chorus or bridge or whatever it is that comes next. If I just open my mouth and expect it to come out, I tend to be doomed to disappointment.