I have been trying to only work on one project at a time. But when necessary, I stack all the pieces of the same size and same color. Cut a piece of cardboard (like from a cereal box) and mark on the cardboard the size of the piece as well as the identification from the pattern (like piece A) and the number of pieces. Then I clip it all together with a bull nose clip (the thick black ones with the little metal clips you flip down). Then I put the whole thing, the pieces, the pattern, a copy of the pattern to write on, into an art bin. Then I mark the art bin with a piece of painter's tape so I remember what it was.
But I do try to only work on one project at a time. If there are too many unfinished projects, I get frustrated.
Good luck!