Treat every stranger with love, respect and a rational level of trust. Of course forget about those who barge into your life full of negative energy, they decide their own welcome the minute they enter your life.
By love, respect and trust I mean genuine feelings, not the superficial flavored things, people who are matured and intelligent get that fake aroma instantly and often become cold and aloof. Whether you believe it or not, lot of people can distinguish between fake and genuine feelings, especially when they see you face to face.
Human beings have a strange strait, they often help others to decide how they should be treated, if you treat an unworthy person with love, respect and trust sooner or later s/he will make you regret that, and then it will be but fair to stop wasting that feeling.
It is far better than losing a good person by mistreating or ignoring him or her, just because you did not liked his hat or sandals that she wore.