Servus!
(Damn it's good to be back!)
Anyhow, a solution that I found is good for heavy traffic is to attach a small cow bell to my bike. Yep, it's annoying as hell but it warns people WELL in advance of your approach. Most folks turn around thinking that a cow is running at them at 25 mph!
I think Kona has marketed something like this for a ridiculous price, check it out.