I nearly always say I'm passing on your left, you have plenty of room. Then I'll often follow with hello, good morning etc and smile. Most of the time I get a thanks and I follow with, "I don't like bikes to blow past me without warning either". Sometimes if there are other riders coming behind me that I can see in my rear view, I may warn them that other bikes are coming as well.
I really HATE the rider that flies by as I'm warning someone that I'M passing without letting me know they are there and blowing past BOTH of us like a maniac. I've nearly wrecked MANY times because of that. The rear view mirror mounted on my helmet helps, but I don't always see everyone when they're coming.