When I knit the Harry Potter scarf for my son a few years ago - I knit it in the round.
Stockinette likes to curl, usually the way that is controlled is by knitting a round or two of rib, garter, purl. To keep it from curling in on the sides - moss/seed or garter a few stitches on each side of the scarf.
If you go back to knitting in the round, use DPN's and cast on fewer stitches to reduce the bulk. You may have to play around with it a bit till you get the desired effect.
OTN - lacy sweater and just beginning another pair of socks - toe up
FO's: garterstitch scarf, dropped stitch cable fingerless mittens, socks for Christmas, and 6 baby hats, cabled toddler hat, hat for mom, MIL socks
My blog: http://knitquiltmom.blogspot.com/
I'm knitquiltmom on Ravelry... I'm slowly loading my projects. I also have several patterns that are being offered as free downloads.