Depends where I'm parked. If I'm at a local bike night, I use a small cable with a lock as a deterrent. If someone really wanted it they could cut the cable but it stops the guys who just pick up helmets and run.
Went to the hardware store and purchased a plastic coated cable with loops. I loop one end near the triple tree and the other end locked to the strap of the helmet which hangs from the handlebars. It’s easier to see than explain, I’ll try and get a picture of my setup. Cable and lock are small and fit in my jacket.