I bought a rubber-like headed mallet at the dollar store. Quite heavy so don't need to whack those unruly seams too, too hard. Works like a charm with a small wooden cutting board under the seam, and a piece of fabric on top of the area to be swatted. This eliminates any marking by the hammer head.
Fine tailoring has employed this system for ages....that's how they get those smooth seams. A wooden clapper works well on clothing, but can be pricey. I find it's easier to sew or quilt over flattened seams and never have trouble with needle breakage, but I don't use a long arm, so that may be the difference.