Repetition
Repetition will teach you new habits. Easier said than done, it sure takes time but there are other tools you can use as well.
Above all, it’s all about Endurance & Perseverance.
Print the lyrics
This immediately gives the song a form, and you can see it. We are very visual individuals, and once you have the form of the tune in front of you,
it is easier to remember it. You can also print out the chords and try playing the melody or accompany yourself. All these technics will help you embed the song within you.
Practice with the original song
Sing-along with the original song you are trying to learn. You have something to rely on, a reference. One important thing is to never imitate the singer of the track. In other words, learn the music and the lyrics, but stay true to yourself when it comes to your voice and your technic. I advise you to watch the video on Finding Your True Voice.
Dictation
Write down the lyrics so you’re sure you know them well. Imagine you’re singing the song in your head while writing down those lyrics. This technic makes it more official that you know the song by heart.
Suggested Video: How To Reconnect With Yourself When You’re An Artist