Oh, this is a hard one! The very nature of St st causes it to curl, so the only way to get it to lie flat is to work a border around it. Garter stitch (knit every row on flat knitting) is very often used, as is a ribbing pattern of alternating knits and purls. If you are going to seam your Stockinette stitch piece, then the curling will be taken care of after it's sewn.