Interleave, learning two or three songs at once, and learn in small, overlapping chunks, in sessions of no more than 20 minutes. The key is frequency of sessions and repeated refreshing. It's vital to allow the brain to forget certain bits, since it will anyway: it's what the brain does with inessential info until it gets the message (through repeated need) that having that info at the ready is important.

By interleaving, even though you might continue doing the same kind of mental work (better to change track entirely), you're allowing the brain to refocus on another segment of material while your subconscious turns over the previous segment. This is more efficient than hammering away at the same song past the point of mental saturation, relying far more on shorter-term memory than on deeper recall. By changing songs, you replace all the contextual cues, like wiping the mental slate.