The simplest and most effective thing to do (aside from dinging a bell) when approaching from behind is to say "Bike back." The people who know what that means will hold their line. The people who don't will treat it as a bell: some will hold their line (or move right), others will look over their shoulder and swerve all over the place, but the latter would do that _regardless_ of how you announced yourself.

This comes from much experience and a little trial and error, and cannot be disputed. <= insert arrogant smiley thingy here.

"Stay" would just make people look around for a dog.