Sometimes what I do is simply say "bicycle behind you" and let them make the decision as to which way they want to move. I have had one incident where two men walking side by side moved in opposite directions, all I could do was laugh.

After passing the people who have moved out of my way, I always say "Thank You!" and most people appreciate that. My girlfriend will ring her friendly bell and then say "On your left". 99% of time it works.