IMO, repetition is only part of the answer to song memorization. The best way to have lyrics solidly inbedded in your head is to develop a personal understanding (interpretation) of the lyrics, and then attach strong memorable 'visual images' of the scene & action (people, place) taking place in the lyrics. Studies have shown that humans are keenly visual, and that attaching 'visual images' an important key to better memorization.

As an entertainer, one of the most valuable skills to have is not only remembering peoples faces, but to remember their name as well. People are always flattered if you remember their name. I can bet that most people here have no trouble remembering (recalling later) people's faces at a party, yet have at least a little trouble remembering everyone's name. Why? Because we usually don't conciously attach something uniquely visual when they say their name when introduced. I've since learned the best way to remember people's name is to conciously say their name 'out loud' to them in acknowledgment, while looking at them, and LINKING a distinct facial characteristic about them while saying their name. This helps LOCK a visual image of what they look like to their name, and has been a great help(for me) in remembering the names of a lot of new people in a room. This kind of "visual linking" works well for song memorization as too, but for this, you'll need to come up with your own creative visual images, in your head.

Scott