I've always checked my helmet either in a Pelican case or one of the multitude of semi-hard flight gear bags Uncle Sam has provided me in the past. Haven't had a problem, knock on wood. Other folks have had luck checking them gateside in soft bags with "fragile" tags attached - gateside checked bags seem to end up on top of the pile in the cargo compartment and don't spend much (if any) time on the baggage machines.
Mike