Remembering songs - I do it in sections. Then as you say, repetition. But do you play note for note & sing a set phrasing, or can you vary it each time a little (or even a lot)?
If you can vary it then it stays fresh for longer & you don't get that "now I've learnt it its done to death" effect.
I've found that the more songs I learn the easier it is to learn each new one. Something to do with the underlying logic of the music I think.

Kris