I sometimes forget the words. Sometimes another band member reminds me. The other trick, as someone mentioned above, is to slip in a line from another verse. The third way is to just improvise something. The improvised lines are usually just banal or cliched. However, most audiences are so used to lines like that, they don't really stick out!
The more striking a line is, the easier it is to remember. I think the better a song lyric is, the easier it is to learn. Story songs are easier to remember, because you have the narrative thrust to drive the song forward.