I try to improve one piece significantly for each piano lesson. So taking it from sight-reading to being able to play it through fluently, or from that to memorization, or from that to working on the artistic interpretation. That way, with each lesson I improve something tangibly. The other pieces I happen to be working on will stand still, but that is preferable to working on four pieces at once and having no discernible improvement with any of them.
So far, I've learned several from the Mompou "Música callada." The Prelude in C from the Well-Tempered Clavier, and a Chopin Prelude in E minor. I have a few Schumann things I'm working through. There have also been pieces I've begun but never finished learning well. I'm learning faster now, so there's that.