Location, location, location. You do not want to be be beside or slight behind an adjacent vehicle. In front or behind. If I'm beside, I have one hand on the horn. I use very loud air horns: just twin trumpet Fiamms driven by a compressor off a 70's Citroen. The compressor is huge. I have my horns mount off a front bumper bolt so there are closer to the grill.
I also ride motorcycles and scooters. One friend wears the most outrageous set of bright plaid pants and has a sticker on his scoot saying "Loud Pants Save Lives".