When piecing flannel, after washing and drying it twice to shrink, I heavily starch it before cutting. My method is to mix a 1:1 solution of Sta-Flo liquid laundry starch and water, "paint" this on the yardage using a large wall painting brush, toss in dryer, then iron with steam. This makes the flannel quite stiff and stable, so that it can be pieced accurately. It's a good idea to use a seam allowance larger than 1/4" if the pattern allows, and I like to press those seams open.