i've never ironed my hakama. but i do what everyone else has stated. i hang it and pin the pleats after soaking/washing, then when its dry, i just make sure to fold it to keep the pleats.
BUT, not to hijack this thread, but what do i do about the knees? i used an iron on patch on the inside of the hakama, but the patch was too small, and now i risk more tearing around the patch
Patching a hakama knee: As a seamstress I am often called upon to do this and I always do it this way:
Cut a piece of matching color/weight fabric 2" - 3" larger than the weak area (so for a hole 1" long but with surrounding weak area actually 2" x 3", cut a rectangle at least 4" x 6"). Press in a 1/2" seam allowance on the patch. Pin it to the INSIDE of the hakama and neatly stitch right along the folded edge, removing pins as you go.
Turn hakama right side out. Topstitch again, 1/4 to 1/2 inch inside the original stitching. Then use a tight zigzag stitch to secure any holes and weak areas to the patch.