Velcro? 
Enrique,In fact, harnesses are typically forbidden at track events, when a car does not have a roll bar.
Hi Joe,Enrique,
That may hold true for Spiders, but hard top like the Milano are able to run with harnesses, and they do help significantly. I find you can drive much better when all you have to concentrate on is steering, brake/throttle, and shifting - instead of holding yourself behind the wheel.
*Another* option is the DOT-legal 4 point system from Schroth.