This weekend I've been reading through a book that got very mixed reviews on Amazon and have decided to agree with the disappointed ones. I won't be buying it. Still, I stuck with it for anything that might change my mind, and after two days of heavy going have this tip to show for it:
If you slip the same stitch every other row, the fabric will fold neatly in that place.
If you slip it on the purl side it will fold in; if you slip it on the knit side, it will fold out.
Probably this is not earth-shattering excitement but as I've never knit a cardi with a hemmed button band or a pleated skirt, it's new to me, and useful for one of the hats I have in mind to make, heh heh.