Not wanting to forget songs

[20yearbreak]

So I have around 15 intermediate pieces committed to memory.  Somewhere between 1-2 hours of material if played straight through.

I realize there's probably some point where you just can't remember / payback every piece you've ever learned.  Or as I've noticed where the quality of performance really starts to degrade on some pieces.  This is where I struggle.  I just want to play back all the pieces I know perfect all the time.  Granted I feel like I can dust off a piece I've learned pretty quick but the muscle memory fades.

I typically only have time to practice about 1 hour a day.  Is there a way to maximize retention?  I am doing something like scales/arpeggios 20 minutes, work on/memorize a new piece 20 minutes then practice 1-2 pieces I already know 20 minutes.

Some days I'll play through all the pieces I know straight through just to make sure they are still there.  At times, I'm surprised I can make it through them as I have no idea what my hands are doing.  Other times I'll think about a certain piece too much while I'm playing and I'll get stuck.  I can normally work through it but sometimes I'll have to pull out the sheet music to get unstuck.

It's almost like it's best to just let the subconscious do it's magic.  It's like how do you determine what is really realistic to retain with the amount of practice time?

I've had a few people tell me not to worry about letting some pieces fade.  It's tough advice to follow though after you've worked so hard to learn a piece.  Sometimes it's tough to let go.

[outin]

Maybe consider if you really like all the pieces? If you are keeping them just for habit you are making it harder to add more great music into your permanent repertoire.

I cannot retain anything in my memory more than a week or so without compromising learning new pieces. So I say goodbye to them and promise to meet them again later some day. Sometimes I will but often not. The same thing I do with people, life just goes on

[20yearbreak]

Maybe consider if you really like all the pieces? If you are keeping them just for habit you are making it harder to add more great music into your permanent repertoire.

I cannot retain anything in my memory more than a week or so without compromising learning new pieces. So I say goodbye to them and promise to meet them again later some day. Sometimes I will but often not. The same thing I do with people, life just goes on

That's a good point.  I have thought about tier'ing things so maybe there's 5 pieces that I keep close enough to where I could walk up to any public piano and play them without thinking.  Then the rest I try to keep them close enough so I could dust them off in a few days if desired.

Maybe I should look at this in reverse?  Rather than thinking about a way to retain better.  Maybe I should work on my sight reading more so I don't care as much if a song fades.  My sight reading is pretty bad.

[outin]

That's a good point.  I have thought about tier'ing things so maybe there's 5 pieces that I keep close enough to where I could walk up to any public piano and play them without thinking.  Then the rest I try to keep them close enough so I could dust them off in a few days if desired.

Maybe I should look at this in reverse?  Rather than thinking about a way to retain better.  Maybe I should work on my sight reading more so I don't care as much if a song fades.  My sight reading is pretty bad.

Sight reading is not quite the same as using the score when playing pieces you have practiced. If you want to learn a piece this way, you will notice that you learn it differently that when memorizing. You still kind of memorize, but with the clues of the score.

 Practicing bringing back old pieces works for me. When I notice that it happens quite fast I am less stressed about forgetting. Of course the pieces never come back quite the same, they tend to be improved

[kalospiano]

I have about twenty pieces in memory.

Originally I used to play my whole repertory every day just to keep it.

Now I don't care that much and I concentrate on a few pieces only, but I still repeat all the others once in a while (be it once a week or two weeks or even more).

Of course the less I repeat the more error-prone a piece becomes, but even in case I completely forget a passage, a quick repetition is usually enough to bring it back in one day or so.

I think you could either try cycling your old repertory practice (one day you only play one old piece, the following day another one, and so on...) or you just dedicate one or two full days a week to old repertory practice only.

[indianajo]

I learn very slowly, and don't sight read very well.
But the never repeat a mistake method I was taught, means the movements of a piece get well and truly memorized.  It was a bodily movement sequence memorized the same way you walked home from school every day, or a horse found it's way around the milkman's route without guidance.  
As I go back to pieces I played 50 years ago, It takes awhile to connect to the memory.  I'll play like I was sight reading, making up fingerings, then after a few plays, I do connect. The old fingerings come back.  I may not be able to play correctly because my flexibility is lower than age 16, and my kinesthetic sense of where my fingers are is much less accurate than 50 years ago.  But the memory is there, I can use the old movements as a guide.  
Don't use your cortex to access physical memories. Your cortex is involved in creating memories, and tends to muddy the recording with new memories if you focus too hard trying to pull something up.
Warning, my memory in general is pretty good for places, objects, words, facts.  Less so for faces & names, although the invention of the internet has led to recent improvement of memory in those knowledge types.  One way I got through school was by flagging the questions on written tests I did not know, and NOT THINKING ABOUT THEM while I did something else.  The answer usually popped up later. This happened to day: we were talking about our visits to Savannah GA before church.  I related a great meal at a restaurant run by some famous television star whose show I never watched.  During the sermon, the name of Paula Dean popped up.  Remembering music is the same way, wait a bit without concentrating (using the cortex) and the connection will surface.  Means I'm no fun at parties, though. Not much instant recall.