Someone asked about a good way to remember song lyrics in this thread :
Good way to remember lyrics when covering songs?
I shan't repeat everything written there .. if you're playing an instrumental some of the techniques may still help!
I've found that just listening to a song a lot so it's settled in your head works very well. Also there's a kind of quantum jump after listening to a song off and on for a week or so.. maybe it's subjective but for me after several days it kind of sits more deeply in the mind and I can get to grips with it more easily.
The other option is to have some cues written on a card, maybe just the structure of the song etc.
That brings me to another point which is to try and visualise the "landscape" of the song - eg is it verse chorus verse chorus wird bit chorus (repeat to fade haha) .. or something more unusual ?
hope this helps.