If the query is for the fastest way to memorize a song, a number of threads have addressed this before, a few times at length. (Mudcat Search is your friend.) I know I've given quite a few tips before, though I don't care to rehash them yet again.

Although repetition is essential, it can also be time-wasteful and even counter-productive if applied in the typical fashion. For greater speed of learning and deeper retention, memorization should be reinforced through a variety of other cues and imagery techniques, and your focus should be on the bits you have the most trouble with (just as with efficient instrumental practice) rather than running through the entire song (or even verse) each time. Ironically, I find the use of formal techniques also helps one focus more on meaning, even when a technique is primarily lexical, since one ends up analyzing the text more closely.