The bottom line for memorization is doing a lot of repetitions, playing through the tune repeatedly. You will probably find that some sections of a tune are faster to memorize than others. In that case the most effective strategy is to isolate the tough spots and do the repetitions until it catches up with the “easy” sections. Don’t under estimate the power of listening. When you listen to the definitive recording of a song you want to learn you are soaking up the way it is supposed to sound and getting it in your head. When you know how it goes it is much easier to play.