Washing and drying a couple of times is good. I starch flannel heavily before cutting because it really helps with accuracy; starching stabilizes it so it moves around less, stretches less, etc. It's a good idea to use 1/2-inch seams because flannel tends to fray, but it's not absolutely necessary. It's also a good idea to choose simpler patterns with bigger pieces; however, starching heavily does allow you to use more intricate piecing patterns with accuracy.
It's actually quite possible to make a flannel quilt without prewashing. I took a class with Harriet Hargrave where she passed around a flannel quilt she had made without prewashing to prove to her customers that it could be done. She measured the quilt before washing and again after washing, and there was minimal shrinkage. Her point is that moderate quilting binds the layers of the quilt together, making the batting the determining factor in shrinkage. In other words, fabric that is closely quilted cannot shrink more than the batting. This was certainly true for her quilt, which was lovely and now mis-shapen at all. Flannel fabrics washed on their own will shrink a ***lot***; I know this from experience! However, if you are willing to machine quilt at least moderately (I'd say something smaller than a large meander -- medium meander or smaller), you can actually skip the prewashing step for flannel.