As far as pre washing goes, make sure you wash it until it stops shrinking. Measure it before you start and when you take it out of the dryer (both wof, and cut length measurements). I've had several pieces that took 3 wash dry cycles before it was done shrinking. The up side is that it will get thicker and softer with each washing.
I lightly starched my flannel backing and had no issues long arming it.