I make two sets of rules for myself when I write:

1. the voice of the narrator (tricks of speech, vocabulary, grammar); his level of omniscience relative to the action; and how much he will tell us of what he may know. In the first couple of drafts, these guidelines are typed on the first page, to remind me (until I get into the groove).

2. the limitations of the locale: weather, distances, demographics, culture. I have a spreadsheet of place names, civic institutions, large populations, etc., to which I refer frequently.

As a reader I greatly resent an author who can’t keep track of his own universe.