The single most important criteria of all is to insure the retreat you select is defend-able. Your defense may simply be its isolation, its physical preparations (Barb Wire, Entrenchments,etc), or a combination of the two. Remember, however, even the most thoroughly prepared and vigorously defended survival retreat may not survive repeated assaults from heavily armed and determined bands of looters in a highly populated area. Conversely, the most isolated survival retreat may fall to a single looter unless basic security precautions are taken.
Basically, stay as far away from the population centers as possible. As food begins to disappear from the supermarket shelves, roaming bands of looters and thugs will gradually begin the attrition process.. The weak and unprepared in the major urban areas will be the first victims of the collapse. The more agrarian and remote cities and towns will be spared the violence initially. However, the interstate highway system and modern communications systems will soon spread the disorder thru most of the country. We will assume that your survival retreat is somewhat isolated, or at least away from the major urban areas.