Following are some tips on memorizing songs:
- Listen to recordings of the song you want to learn. Whether you listen to various versions to explore the possible interpretations or if you just have a favorite version you check out over and over again, listening is a significant and important way of first memorizing a song. Listening gives you a chance to internalize the song without worrying about what the details, and over repeated listening you can memorize the soundscape of the entire tune, changes and all, without having a clue as to what the notes are. When you approach learning a tune this way you'll be much more sure footed and confident once you tackle the actual notes.
- Sing along with the melody on a recording. Then sing it all by yourself. If you can do this you've probably done more than half the work of memorizing the melody already.
- If you have time transcribe the melody, especially if it's a song you've already internalized through listening. This will help strengthen your ears and make the process of memorizing tunes faster and faster in the future.
- Memorize the melody on your instrument and play it with the record without looking at written music. Finally, play it from memory all by your lonesome.
- Again, if you have time transcribe the changes. If you need a process, start with the bass notes and then determine the chord qualities.
- Memorize the changes and improvise along with the record without looking at written chord changes. Now, practice improvising through the changes with only a metronome.
- If you're looking to further internalize the tune, I would suggest transposing it (in your head, and by ear) into a few different keys. If you need to internalize the melody further transpose the melody, or if you only need to better internalize the chord changes just transpose those. For this exercise I would suggest thinking about the harmony functionally, so instead of a CMaj7 chord in the key of C think IMaj7 and instead of Dmin7, G7 think ii7 , V7. That way you can more easily transpose to any key. What's the ii7 chord in the key of Gb? Even if answering that question isn't easier than transposing Dmin7 up a diminished 5th understanding the harmony of a tune from functional perspective will help you internalize it on a deeper level and prepare you for mass transposing on the spot.
These are all things that I've done in the past that have helped me memorize tunes. I rarely do everything on this list for a single tune. In fact, I probably only do a few for each tune I learn. The most helpful technique I've discovered is repetition, playing the tune over and over again. If you have any suggestions feel free to leave them in the comments.