i don't do much piecing with flannel outside of backs, but those are loooooooong seams.
i use starch on both sides of the fabric, and make sure it's good and dry before handling it.
then when i sew it i am extra careful to press, not iron with sweeping motions.
i love flannel and i think it's great to work with, but it is highly mobile. not sure i would take it on in a pattern that required cuts on the bias. it just wriggles too much!
good luck!
aileen