it sometimes cannot be avoided unless you're one of those freaks that learn it once and it's done for life (like they can learn an entire concerto in short order and it stays in their repertoire forever basically, rare but those mutants exist/existed).
if you have works you feel are worth keeping in rotation, try simply keeping them in the 'periphery' of repertoire, that is perhaps play through it ever few days, a couple times a week give i a good ol college go and 'perform' it, once a week practice it, give it a break for a week or two completely then use it as a daily warm up, etc.
just try playing around to see how little you can play it before it starts to fall apart then just barely inch back a bit on frequency so you're right at that line where it stays put together.
i liken the approach to what works well with ,you guessed it, exercise diet and leaness. rather than trying to be stay ultra muscular and defined year round, i will peak, then just relax a bit (but not let it get out of control, that is i stay to where i can see my abdominals/six pack albeit not razor sharp), then i know i'm not more than a few weeks of slighltly tighter dieting to get back to where i was instead of months.
for piano, if i need a piece like that, i'll let it slide to where i know a few weeks of carefuly practice can bring it back in short order vs complete loss of the work (rare i have so much i want to play and i practice so much usually when a piece is done, it is DONE, like for years before i'll revisit).
sorry for tha chatty resp. there's probably something at least mildly useful in there somewhere.
Topic: Maintaining repertoire