Your County Extension service might be able to help and advise you on this. As Daisy said, Master Gardeners here host a Gardens tour each year that is open to the public. People who know the various Master Gardeners have also offered to put their gardens on the tour, then a whole lot of people get to spend a day going from house to house, viewing gardens. This way you will get people who are genuinely interested in gardens and gardening, not crazies and criminals.
If you just put out an ad in the paper, or (heaven forbid) somewhere like on Craig's List or another public forum, you could get a whole lot of very strange people trooping through your yard. I'll take uppity old gardeners any day to nutcases who want to case your house.
