I think that is the secret George, small but frequent refills. A bit of overflow keeps it pretty clean.
When I first did this bird bath thing it was really amusing.
The birds would come, drink daintily, then get in and splash around a bit. They would then perch on the edge, turn around and poop into the water.
For the first couple of weeks the thing was just full of bird crap. But they learned very fast. These days there is almost never any crap in there when I scrub it out, probably about every two weeks.