When knitting flat you can wrap the yarn clockwise or the "wrong" way on that first purl. When you encounter the twisted stitch (it'll be a knit) on the other side you knit into the back of it. Here's the site I learned it from.
I tried knitting normally, but wrapping the purl clockwise when knitting in the round and then knitting through the back loop of the purl on the next row, but that didn't work. It seemed to make it looser.:??
It tends to show less the more ribbing you do, but it still bugs me.