If you’ve got one of those helmets with a ratchet strap fastening, this might be useful for you: a strap that connects to the two ends of the buckle to create a strap that you can sling over your shoulder.
If you’re feeling protective about your helmet, you can always pop it into its helmet bag too, and it goes without saying that you could use it to carry a spare when you’re going to pick someone up.
And if you haven’t got a ratchet buckle? Well, you’ve got a project this weekend to recycle an old webbing strap and find an appropriate buckle, in which case remember to put a broader shoulder strap on it too, to spread the load.