I heavily starch flannel after it is washed. This makes cutting and piecing much more accurate because the flannel doesn't stretch or distort. My method is to "paint" a 1:1 solution of Sta-Flo liquid laundry starch and water on the yardage, toss in the dryer, then iron with steam. Comes out with the stiffness of light cardstock.
It can be a good idea to use 1/2" seams with flannel because it ravels more easily.