I suppose, given time, rote repetition playing the same notes repeatedly will yield results, I believe a far more effective & everlasting approach is to hear & understand what's going on musically while you're memorizing a tune.

This requires understanding the basic 'fundamentals of music, of which includes the ability to recognize (by ear) and play at will (on your keyboard), the basic chord progressions (I-V7-1; IV-V7-1; IIm-V7-I; I-VIm-IIm-V7-1; etc). Another important music skill to develop is note interval recognition & playing/singing them at will too. This includes the ability to instantly recognize/play all possible melodic 'note intervals' (both ascending, descending) and being able to recognize (by ear) & play basic chord types (minor, major, aug, dim, 7ths, 9ths, b9's, etc). Once you've developed these skills, you'll be able to not only memorize songs quickly, but play songs 'on the spot' by ear too.

Lyrics Memorization: Here's a link to a previous discussion thread on that.

Scott