Not a weird question, truly.
I don't know the secret except relentless immersion in the material to be learned. In the car, in the house, in your headphones when you're studying it. Plus, memory of music happens far more readily when you have an emotional connection to it; as you listen to each song and make a chart for each (which I find essential when there's any large quantity), write down thoughts in the margins or at the bottom of any sort of emotional/personal resonances you feel about it. If you can't think of any, find some.
It's potentially a big help.
It does come naturally to me, but that's just because I've learned so many repertoires over the years. It gets easier, not harder, in fact.