Your helmet is likely to be the most precious and expensive piece of equipment you own. The thought of it getting banged around in your suitcase while loading and unloading the plane probably makes you a bit nervous. Here's a pro tip: bring it on the plane with you. As previously stated, most companies allow you a piece of carry-on and a personal bag (eg: a backpack and a laptop bag). Take full advantage of it and take your lid in the cabin with you.
You can either carry it in its pouch or safely pack it in a carry-on bag (duffle bag, backpack, etc.) to have space to store other personal effects and have all your precious and important gear and documents with you at all time.