1) Observe
Record yourself singing or speaking through the lyrics without stopping (unless you’ve forgotten the rest of the song). Then go back and take notes of what was remembered and what wasn’t. Look for the areas of the song that have no or few repetitions, and lastly the parts of the song that are the most musically dense (for instance, having lots of runs or very quickly sung lyrics). Really take your time on this one, especially if you’re feeling nervous about it.
2) Decide
Decide among which of these is your first priority to begin memorizing, and ignore all the other areas that are repeated often (like the chorus) or easy to remember (often the first verse of a song). Once these areas are solid, then move on to the more repetitive parts of the song.
3) DO NOT start from the beginning!
When you practice memorization, don’t restart from the very beginning of the song every time you have a lapse in memory, only go back to that section you’ve been working on.
4) DO put yourself on the spot!
Random reinforcement saves the day! If you only check your memory of a song in the middle of a long practice session, the work will not be as effective as quizzing yourself throughout the day when you are in the middle of another activity. This is the true test of memory and how you build trust in your memory of a song. So put yourself on the spot, before you’re on stage.
5) Use goofy gestures and your imagination.
If you keep forgetting a word in a song, let’s say the word “cry” do a crying gesture or sad face right before you get to that word in the song. No, you don’t have to do it in performance, but physical interaction gives you a more obvious way to remember a particular lyric in a song. 
6) Most of all, a reason to remember.
When you’re pressed for time, don’t forget the purpose of singing! To express a message! I often found that if I thought about and gave my feelings free reign in expressing the words of song through speaking them aloud and singing, it was much easier to remember the song, because I had a reason to remember.