I have started employing a practice I learned in the Post Office. When approaching I say in a very loud and cheery voice, "Mind your backs, please, mind your backs [ladies and gentlemen*]" repeat until they move and then say, 'Thank you very much, [Ladies and Gents**]."

It pretty much always works to move them out of your way if you say it loud enough and usually the cheeriness means they don't feel awkward. The trick to it all is to keep walking once they've noticed you.

*Delete as appropriate

** Again, delete as appropriate.