Sounds VERY familiar!!!
I knitted a cable cardigan with a hood from Aran wool over the summer and realised I coudn't get away with my usual back stitch for the seams on this one - my little grand daughter would look as if she had mountain ridges going up her arms and down the sides.
I looked up mattress stitch, - tried it, got it right (somehow) and saw what a neat flat line it made. Yahoo!
All the Best