TL;DR: Do not read as you practice. Visualize everything.
I used to consider myself hopelessly bad at memorizing lyrics. I thought this was because of some problem with my brain. Turned out I just didn't know how. Here's how I changed my fortunes.
There are two parts:
The most important thing in this guide is this: You should spend as little time as possible reading the lyric sheet.
Amazingly, you can sing a song 100 times while reading a lyric sheet and still not be able to sing it by memory.
Reading over and over again won't help you memorize. My pseudo-science explanation is that if you are reading, your brain has no reason to try to retrieve the lyrics by memory and so it won't bother. What we want to do is train our brains to retrieve memories.
Pseudo-science aside, what works for me for memorization is practicing recalling the information.
Say you are the singer and you've got a first rehearsal with the band coming up. You want to have the lyrics memorized in advance. Do not think that it will be better to read from the lyric sheet for the first few rehearsals. You will be robbing yourself of important retrieval practice.
You can pay the piper now or later, but at some point you have to do the work to memorize the lyrics. It is much, much better to do this before the first rehearsal if you can.
Once you've mostly "got" the lyrics, it will be easy to practice any moment you think about your upcoming performance. You can practice outloud or just in your head in fast forward. You can practice in line at the hardware store or in the kitchen waiting for toast. But this kind of practice can only kick in once you have some part of the song at least mostly memorized.
Let's say a line is on the tip of your tongue. Your impulse is probably to just immediate look up the lyrics and solve the mystery that way. But then you aren't giving your brain the chance to train itself to retrieve the information. As you search your memory, you will naturally develop associations and hooks that will help you find it later. For example, you'll be thinking about what the song means or some image in the song, or thinking about the rhyming pattern and that will lead to your finding it. When that happens, your brain now has a better chance of finding the memory next time since you have trained a route to get there.
You want to think about what the lyrics mean. Some of the most difficult lyrics to remember are lyrics that are very abstract or sound like nonsense, even if they are beautiful. The Boot Scootin' Boogie is easy. Mr. Tambourine Man is hard.
If there's any kind of story involved, you want to be aware of it. Imagine the progression of the story in any way you can. Think about the overall arch.
Visualize anything you can. If there's a flower, imagine the flower. If there's a river flowing, picture it clearly.
Your mind will naturally visualize. Pay attention to what comes up. But, also, actively think up things you can visualize. Ideally every line should have some kind of picture associated with it.
We talked about noticing and visualizing the story, but it also helps to notice just about anything about the lyrics. For example, anything interesting about the rhyming pattern or the way the sounds of the words go together. When we do this we are creating memory "hooks".
You may want to experiment with singing along with the recording of the song as you practice. This is fine, but know that you are getting cues from the singer or even the instrumentals that won't be there when you need to sing by memory. You can easily be singing slightly behind the singer on the recording, which is almost like reading a lyric sheet so it can be ineffective practice.
If you don't know any of the lyrics, start by reading or listening to the whole song. You want to get a feel for the overall story. Then, focus on the first verse. Or maybe just the first two lines. If you can get those first two lines memorized, that's a win.
Once you mostly can sing a verse or the whole song, you can practice in the kitchen, in the shower or even just in your head.
Don't expect to memorize it in one session. It's better to do it in several spaced sessions.
Once you have it memorized, you'll want to continue to spaced sessions to reinforce the memory. You can gradually have longer and longer spaces between the times you sing the song (by memory). It's definitely "use it or lose it", but you don't have to use it too often for the memory to stick around.
If you want a sure-fire way to develop a repetoire of memorized songs, join us in Sharpen the Saw. There, we practice skills every day and it would be great to have you as part of the group.