If you are sewing more than a couple of extra long strips together you are supposed to sew one one way and the next the other way. Start at the "top" on the first two you sew together and start at the "bottom" when you add the third strip. This is to help prevent bowing.
Personally, I am a huge fan of starch to help things hold their shape. I like to sew fabric that has been starched into cardboard. It helps me.