If a b&m sells liquor, then I buy drinks and don't worry so much about buying their cigars. If they don't, and I'm going to smoke there, I'll buy a couple of cigars. If I know the owner or the guy working, I'll often give him a cigar or two, as well.
In this hobby, etiquette is usually little more than common sense: don't be an ingrateful, cheap-ass, mooching, expecting-something-for-nothing douchebag and you'll usually be ok.
