Being polite and respectful goes a very long way most of the time. Other times, it is best to simply avoid places/people that are prone to violence at all times.
There are a number of "bad" neighborhoods in NYC (Brownsville, BedStuy, Washington Heights, South Bronx etc.), where if you walk around at the wrong time of the day, you will most likely get mugged or even worse.
Best thing to do if you find yourself in those places is to keep a neutral face, watch your step, be courteous and give the right of way to others and, if approached, be polite.
If you happen to be at a bar where some idiot decides that he wants to fight you for no reason, simply tell the individual that you are not going to fight and excuse yourself towards the nearest exit.
I don't believe in "tough" guys. You either are skilled and capable of defending yourself, which most often leads you to never want to fight or you have an unfair advantage (gun, knife, friends), which leads you to act in an obnoxious fashion (i.e. tough).
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