Hmm I think it depends on the person Im going to pass. If they are older and look like they might get a fright I try to call out good morning or something from a bit of a distance and then say hello as I pass. Sometimes I use on your left and sometimes I use the term heads up. This seems to work well with non riders who dont seem to know what on your left means (sometimes they move to the left - yikes
).
The most effective way to do it, is to do it.
Amelia Earhart
2005 Trek 5000 road/Avocet 02 40W
2006 Colnago C50 road/SSM Atola
2005 SC Juliana SL mtb/WTB Laser V