Do we need to be a shmata when dealing with difficult people?

What are some strategies and tactics for dealing with difficult people?
Can you reciprocate the negativity?
What if it’s your boss, Rabbi, Rosh Yeshiva or spouse?
When does the prohibition of hating someone apply? What if he earned the hatred? 
Do people get crankier when they get older?
How can someone tell if they are a difficult person?