Which is it, Southern or southern?

Are they any English majors or grammar experts out there that can help me out? I am have been having trouble deciding whether the word Southern or Southerner should be capitalized or not. I was informed by a professor in graduate school that the proper use is, southern, with a small "s". Later, I was corrected by yet another authority that the "s" in Southern or Southerner should be capitalized.

I know that the word South should be capitalized when talking about the region, and should be lower case when speaking about the direction (same goes for North, or course). But when it comes to Southern it seems that anything goes...although there must be a correct and incorrect way to use it. I have often referred to my multitude of books on Southern history for examples, and there too it is used both ways, although, it seems that more recent texts go with lower case "s," while the older books use Southern or Southerner more. It seems that the only rule I can figure is to use one or the other consistently.

If anyone has an opinion or actually knows the rule I would be glad to be properly informed. I have always used Southern, Southerner or Northern and Northerner on the blog here, but I would like to be correct. It will help me sleep better at night.