There was another thread recently that dealt with ways to get the word out. You might want to search around for it.

It sounds like your doing the right things. Pounding the pavement. I know it is a frustrating exersize.
Do you have literature that you leave at your stops?
How about offering a "Free checkup" as an incentive?

You say you have several satisified customers already. Maybe they can help spread the word.

Just off the top of my head