How to have a peaceful mind?

In the Bhagavad Gita, Krishna many times talks about the equanimity of the mind. He says we have to treat success/failure, honor/dishonor, pain/pleasure equally and such equanimity of mind is called yoga or mind control or mindfulness. 

To do this, 

1. We have to learn to tolerate both good and bad. If we become too happy then naturally we will also not react well when things go wrong.
2. We also have to refrain from interpreting incidents in our life as gain or loss. When we qualify something or someone as favorable (gain) or unfavorable (loss), then invariably we will fall in the trap of duality of happiness or sadness. 
3. We have to spend time understanding that my life, my family, my job and my assets is a result of something more than just me or my efforts. No doubt, because of our hard work, we reach a certain socio-economic status but it is incorrect to think that I am the sole reason for my success or failure. There are other moving parts that we do not control and those parts either assist or desist our progress. So having a broader vision for life is necessary to understand our own progress.
4. We have to be grateful not as a matter of political correctness but genuinely from the heart understanding truly the assistance we received. In this way, we must be grateful to mother nature as she is providing our needs, great sages who have given us spiritual wisdom,other beings such as animals who help us with our needs,family and friends,other humans who participate in our life,and parents/grand-parents/fore-fathers without whom we cannot be where we are. We have to acknowledge and be anxious to return the favor we receive.
5. Finally, like it or not, this creation exists due to a super intelligent being whom many call God or universe etc. Whatever superlative we want to use, we have to accept there is a grand design in this universe and without that we cannot do anything. To the ultimate designer, with a pinch of affection and love, we doff our hat!

By having an all-encompassing thought process (aka mature) about life with all its variety, we will only be centered and not skewed one way or other. This centeredness or equanimity is essential for having a peaceful mind. 

Hare Krishna