The cagers will always claim they didn't see you, no matter what you're doing when trouble happens, but to counter that I have a lot of reflective material on the bike itself, I wear a high-vis safety vest, reflective wraps near my ankles to keep my pants out of the chains and have a flashing LED light on the back of the bike and the back of my helmet.
For those that still don't notice and cut me off, I love the AirZound air horn system
. Up to 115 db, I keep the adjuster at about 3/4's on mine and no problems yet. I also wear steel-toe runners when I ride, and anyone that crowds me gets a good kick in the bodypanels.