Good question with a lot of interesting answers.

I try to use "on your left" or "behind you" with bikers, and with walkers/joggers I'll say something like "coming up behind you" or something along those lines. If there are more bikes behind me, I will say "and there's 2 more coming" or "one more passing, too" when I go by.

If someone passes me and says "on your left" (or anything), I *always* thank them for it -- may as well encourage them to keep doing it.

The scariest unknowns seem to be kids and people with earbuds/headphones. When I'm coming up behind a big group, a kid, or someone who seems uncertain, I slow WAY down, approach with caution, and try to gently warn them away (sucks to slow down, but I'd rather be safe than take someone else or myself out)!

Sometimes I ride with my husband and/or friend, who both have bells, and we use them when we are approaching which often makes people look around and think about people behing behind them. I usually try to say something anyway, at least to be friendly and thank them for moving.

As an aside, my dad was joking about the "roadie nod" one of the last times we were biking. He's been biking in a tshirt and regular shorts with cycling liners (so, not very visible that he's biked much, even though he's on a road bike), and feels like hardcore roadies coming toward him and passing by don't give him the same "nod" and wave they give me in the "lycra and jersey uniform" He says something to pretty much everyone we pass (joggers, bikers, rollerbladers, people on the sidewalk) even if he's not leading. He definitely likes to be the bike ambassador