I have started saying, "Have a blessed day." Just today, beginning with the cashier at O'Neil's Market. We'll see how it flies. I'm thinking it could be just the wedge I've been looking for. I got the idea when a postal worker who weighed my package said, "Have blessed day," as we finished our transaction.
Verbal expressions mostly creep into the English language unawares, but I happen to remember the first time I ever heard the leave-taking "Have a nice day." It was 1972, which I know because that was the year I lived with Karen M. at college, and she was the one who said it. I thought it was dumb (though I liked Karen, who was one of those gentle hippies). Dumb because empty, I reasoned: The speaker has no power to grant the hearer a "nice day." And dumb because "nice" is a vapid idea. (But did I mention that I like Karen, and I was a critical boor in those days.)
I still have trouble with "Have a nice day," and rarely manage to choke it out in social intercourse (I usually resort to the older "Good-bye"). But as long as we're stuck with that unfortunate figure of speech, I'm thinking that "Have a blessed day" has potential. I'm getting excited about its manifold benefits as I write: It signals that you are a Christian to another stranger out there who may be a Christian-like a secret handshake. It is not overbearingly evangelical to those who are not Christians, and yet opens the door for further discussion if they so choose.
I will try it out for a few weeks and let you know what happens. I figure if people are not embarrassed to say something as lame as "Have a nice day," I shouldn't be embarrassed to say "Have a blessed day."