You could try doing garter stitch at each edge to discourage the curling...like this:
CO however many and do
Row 1: K5 [K2 * P2, K2 *] k5.
Row 2: Knit to end of row
Repeat 1 & 2
Depending on what type of yarn & needles you're using, you could do 3 (lg needles, chunky yarn), 4 or 5 (smaller needles, worsted weight yarn) garter sts at each edge.
Also, check this
series of posts from the How to....forum.
Hope it works out for you.