Always learn a song from the same page of lyrics. If you make notes/changes, don't reprint/rewrite the page, carry on using the same (messy) page; those distinctive notes and alterations will aid the visual memory.

For long, complicated songs, stick in an instrumental break or two (or if, like me, you can't do anything fancy on the guitar, a couple of hummed lines). This will break the song down into manageable chunks.

With some songs, each verse lends itself easily to creating a separate mental picture for that verse; for example, Paul Simon's 'For Emily', or Phil Ochs's 'Changes'; once you have the pictures, you still have to remember the order of words in each verse, and the order of the verses, but it will be much easier.

Other songs can be trickier. When I learned Elvis's 'Love Me Tender', which has quite simple lyrics but in which each verse and chorus have the same pattern, I made pictures using the key words of each verse: sweet-go (a sweet being thrown away), long-heart (an elongated heart symbol) and dear-mine (a deer looking into a surface mine; originally it was looking down a shaft mine, but I became a bit concerned that my imaginary deer might fall in!).

When performing - whether in public or alone at home - if you forget the words, it's usually worth taking a 'run up' to the forgotten words, restarting either from the line before, or the beginning of the verse. Try that before checking with your written page. (Don't start the whole song again, though; you're more likely then to get stuck in the same place, and if you're singing in public, your listeners may find it a little boring!)