I like to starch flannel heavily before piecing it. My method is to mix Sta-Flo liquid starch 1:1 with water, then use a large wall painting brush to saturate the fabric with starch. I throw the fabric in the dryer, then iron with steam. Flannel comes out quite stiff this way and no longer stretches. Makes for accurate piecing.
If you are making a rag quilt, a 1:2 mixture of Sta-Flo and water would probably be enough.
Also, when sewing flannel, I always use a 1/2-inch seam instead of 1/4-inch.