I agree with Prism's suggestion for the starch solution. I use the same. Spray onto fabric, let it soak into the fabric, then press. Repeat as necessary. I press at each stage of construction of a block. I press the seam as it's sewn, then open up the block and press again. I press my seams open to cut down on bulk but that is certainly up to you. You can always starch again when pressing your HST's after they are cut/sewn to help with any further distortion. The starch is all going to wash out when you wash your completed quilt so no worries there.