Carrying an Extra Helmet

I spent some time with my 7 year old nephew the other day, and when it was time to take him home, we took my bike. After I dropped him off, I brought his helmet and gloves back with me. Carrying his gloves was as easy as slipping them in the pocket of my vest. His helmet was more of a challenge using the chin strap to cinch it to the back seat. It worked for me just fine, but I'm a little dissatisfied. I've seen plenty of other sport bikes carrying an extra helmet that looked seamlessly attached to the sides of the bikes. I really want to know how they do it.