I use the Helmetlok on my bikes. I set the helmet on the mirror and lock the strap "D" ring to the mirror staff. This keeps the interior of the helmet dry if it rains while I am inside the restaurant or store. I used to hang the helmet upside but wearing it wet is not a fun as it looks on TV...
Keep in mind, anyone can always cut a strap to steal the helmet, I have been lucky over almost 40 years now.
Thread necro, sorry! After all this time, has anyone tried the lamonstergarage lock mentioned at the beginning of the thread? Seems a lot of money, but does sound A LOT better than the gun lock (Using the gun lock myself now)
I use this... Amazon.com: Master Lock Cable Lock, Set Your Own Combination Bike Lock, 3 ft. Long, Assorted Colors, 4605D: Home Improvement
FYI - Shoei Neotec II Splicer as it may depend on your helmet as to what lock you choose.