Much like Ameria, when I learn a song I play it obsessively over an over, doesn't matter what Language, although When I wasn't familiar with Japanese, I looked up lyrics and had those in front of me so I didn't learn them incorrectly.
That’s still a good idea even when you are.
Also, if you're familiar with Japanese and there are NO lyrics, best way to learn is write 'em down. (Like taking notes in class. If you write 'em it's hard to forget. Yay Subliminal mind!)
But you really need to be familiar with the language for that, some things are hard to tell the difference of if you don’t know the sounds and places of different words.
afterwards, to get yourself off the lyrics, there isn't any real trick other than to learn the order. Translations can help that, or if you just have a knack for that.
As long as the first word in every verse is right, it all just comes.
Might not work for everyone. (We're all different people after all)
I never had any more trouble with Japanese than English (It's just…always been there...By sister and me boppin’ along the Cutey Honey and EVA themes when we were...Four. Thanks dad.)
Again, I’m just a singer, and don’t know about anything else.