I'm thinking, having worn full-faced motorcycle helmets, firefighting apparatus, and gas masks, that the fan is likely single best solution.
In the meantime, as someone stated, wearing your helm in familiar surroundings (hanging out at the house/garage/etc) is a great way to acclimate yourself.
Not sure if this will help, but, it's a trick our Platoon Sgt. from Recon taught us; when you start to get the sensation, before the actual claustrophobic state kicks in, slowly inhale for 4 seconds, then, slowly exhale for 4 seconds. Do this several times, trying not to hyperventilate. Slow, steady streams of air. Usually, about 10 times should do it. Concentrate on the breathing and the counting.
While it may take some getting used to, I assure you it works.