Remembering lyrics can be hard work, and each person responds to things differently, so it really depends on what works for you. Start by reading the lyrics out loud to yourself and then consider them both by themselves and with the music. In order to make them stick, you have to make your own detailed analysis of what the words mean. Avoid trying to memorise too much in one go; concentrate on one page at a time.
When you know the lyrics a bit better, a good idea is to walk around singing them so fast that you’ve got no time to think, so they become an automatic response. The music won’t let you stop and think while you’re performing, and there will be a whole lot of other things happening on stage that can make you forget what you’re doing. So repeat the lyrics while doing something else, such as throwing and catching a ball, walking round the supermarket, cooking or doing the dusting.
If you find that you’re forgetting certain parts of a song, work out which lines you tend to forget and look for some kind of pattern. It can be as simple as an alliteration, such as the two Ws in the line “When I am with you”, or a pattern of ideas, such as the similar sentiment of “on my own” and “all alone” in On My Own from Les Misérables: “On my own, pretending he’s beside me/ All alone I walk with him ’til morning.”
When people forget lyrics, the problem is nearly always that they haven’t been clear in their mind about the story they’re trying to tell. By making sure you know exactly what story you’re getting across, you can solve this problem. On stage you can also use your location as a physical prompt: when practising Joe Gillis’s song in Sunset Boulevard, the cast found it useful to have a different physical position on stage for each phrase, so the song was ingrained in their muscle memory and they could remember where they were.
Even once you’ve learned the lyrics and have been singing them over and over again, try to return to them from time to time to refresh your understanding of what they mean.
Source: http://www.theguardian.com/music/2009/may/10/tips-stage-performance-singing