There has been a lot of discussion on here about singing songs from memory as opposed to a lyric sheet or ipad.
Briefly summing up, gawping/squinting at a lyric sheet or ipad destroys spontaneity and contact with listeners, forgetting song lyrics you thought you had memorised can lead to an awkward gap while the download catches up and losing the flow or even "Oh, I can't remember that one I'll do something else".
I'm now getting to that age where committing words to memory is increasingly hard but I'm still trying to do it on principle.
If you think you've memorised a song but it's a new one or one you haven't done for a long time, what about giving a copy of the lyrics to another singer sitting next to you and prompting as in a play.
In a dramatic production this is used effectively to keep the flow of the play going. Often it can be just a whisper of a single word.
I tried this quite successfully last night with a long song that I hadn't done for a long time and got through it with a couple of one word prompts.