The porch is a place to stay cool. Porch society is dying off as home air conditioning units become ubiquitous in American homes throughout the South. People can now stay indoors and still be cool, and need not interact with their neighbours. This is progress, but it is a sad thing. But porch society still exists, and it should be carried out properly.
THE PUBLIC AND THE PORCH
It is customary for the (potential) host to greet those that he or she knows if they are passing by. The (potential) guest should not take a greeting as an invitation to enter. The (potential) host may say “Why don‘t you join us?”. This is a pleasantry, and should be politely refused by the (potential) guest. If the (potential) host genuinely wishes to invite a guest, he or she will repeat the invitation.
BOUNDARIES OF THE PORCH
The guest should be aware that an invitation to the porch is not an invitation to any other room of the house, save, possibly, the bathroom. The host should remember that the porch is a social area. If you require privacy, the backyard is the place.
The guest should not expect refreshments to be provided. He or she should certainly never ask for refreshments. If the host has refreshments on hand, he or she should offer them to the guest.
LEAVING THE PORCH
The host has three good ways to indicate that it is time for the guest to leave. 1) Say "Well-". The guest should thank the host, and leave. 2) Stand up, and stretch. The guest should stand, and leave. Keeping the host talking while standing is frowned upon. 3) Ask "You don't have to run, do you?". The guest should declare that they do have to leave, and go.
Lake Wobegon Days by Garrison Keilor, attributed to Etiquette Along The Mississippi by Gaylord Gibbons.
An article in the London Review of Books, currently lost.
Novasoy informs me that this is also applicable in Kentucky. Jethro bodine says this is applicable in Georgia too. Kensey reports that this is applicable in some parts of Virginia too, the ones that have not been citified like Washington D.C. miska adds Kansas and North Carolina to the list. If you know of any states where this guide is true, please /msg me.