You might try starching your material before you cut. That's something I learned from this board. It helps keep your material from stretching. You can use Mary Ellen's Best Press, store bought spray on starch, or make your own starch. (There are lots of recipes on the board. If you search under starch I'm sure you'll find them.)
Also, do you press or iron? When working with cut pieces of material, make sure you put your iron down on one place then lift and set down on another place. Do not slide the iron, as this can cause stretching also.