Thats interesting, I don't know that I have ever really thought about trying to remember a song. I mean generally by the time I figure it out I know how it goes pretty well.

I don't know how you play and if you have been playing a long time you have been playing a lot longer than me, so I don't want to be presumptive, but try hearing the song in your head as your playing. Thats generally what I do I think. Then you just play what you hear rather than trying to "remember" fingerings or anything. I'm pretty sure this is what I do. But I have never really paid attention.

If I don't play something for a long time I tend to forget it. If you think it is actually a problem with your memory you could try to play those memory games or take vitamin supplements or whatever. Maybe Zinc?