So, I got a "wheel roller" from Louis.eu a while back, so I could drive the bike onto it and rotate the rear wheel (cleaning etc) without hoisting the entire bike. That's a good little item in its own right, but what struck me with a "holy crap, how did I not think of that before?" thing was an accessory to that - a simple velcro band you wrap around the front brake lever and the handlebar to keep the bike from rolling.
Why didn't I think of that before? If I ever have to park or stop the bike on an incline, a simple velcro band worth nothing in the pocket can create an instant parking brake (just, you know, not when the front disc is very hot or it may take damage...)