Short pants are usually not permitted inside the public institutions (hospitals, etc). Wear modest dress when visiting monasteries.
At beaches, taking off the bottom part will likely create a stir, and is generally reserved for designated nude beaches.
When toasting, look a person in the eye while you toast. Also remember that being obviously drunk is a sign of bad taste and worse character in Montenegro: You may be invited to drink gallons, but are expected to be able to hold your drink.
Do yourself a favor, and DO NOT get yourself into an argument about Montenegro politics and/or division between Serbs and Montenegrins. It's way too complex, and you could hardly understand Montenegrin political and social issues by reading a few (possibly biased) articles about it.
Also, do not throw garbage on street or somewhere else, it's extremely impolite.
weybridgewanderer