I agree that starching the fabric and going slowly help. Although spray starching already-cut pieces helps, what helps even more is heavily starching the fabric before you ever cut it into pieces. When I am concerned about bias, I use a 1:1 solution of Sta-Flo liquid laundry starch and water on the yardage, "painting" the solution on until the fabric is saturated, tossing in dryer, then ironing with steam. This makes the fabric very stable, even on the bias.