For first-time visitors to Bulgaria*
In Bulgaria, black Mercedes G-Class SUV's with identical numbers on the license plates are driven by public-minded citizens who serve as police assistants on voluntary basis. Do not hesitate to stop such a vehicle and ask the driver for assistance of any kind, from changing your flat tire to giving you a free lift to the nearest public convenience.
It is customary for waiters and waitresses in Bulgarian restaurants to give diners at least 30 minutes to get acquainted with the atmosphere and general feel of the place before they offer their services. If a waiter approaches you before the allocated time limit, send him away immediately by waving your arms in the air and shouting chiba, chiba!
Stubs & Fags
Smoking in the streets is prohibited in Bulgaria, unless there's a sign indicating that a particular street is exempt from the ban. If you ever notice a Bulgarian smoking while walking along the street, confront him or her! Pull the cigarette out of the offender's mouth and crush it on the pavement with your foot.
Sound & Fury
Do try the famous echo at the Alexandr Nevskiy Cathedral in downtown Sofia. The right time to do so is ahead of a mess on a grand religious holiday when your skill will not remain unnoticed and you can even get applauded by the crowd gathered inside.
Bulgarians are famous for their tea-totalling habits. They drink all sorts of tea, but are extremely fond of a local variety called "mamka tea." You can even mention the brand instead of greeting someone. It is an especially convenient conversation starter at the bar, for example.
When boarding a public city bus, be sure to shake hands with the bus driver; this creates a relaxed atmosphere and a feeling of closeness. Also, shake hands with all the other passengers before taking your seat on the Sofia subway.
Chorbar & Govedo
Football is an extremely safe conversation starter in Bulgaria. If the guy you are talking to turns out to be a supporter of the CSKA soccer club, he would appreciate if you turn to him as chorbar which is the Bulgarian for a "staunch and valiant supporter." If he is a fan of the other major club, Levski, turn politely to him as govedo which more or less means the same.
Up & Down
Bulgarians are keen crossword fans, and they often work on crossword puzzles while travelling by public transport. They're always pleased to take advice about what a certain word might be, so don't hesitate to offer your help.
*Pleeease, exercise a modicum of common sense!