The name of the twelfth chapter of Shrimad Bhagvad Gita is Bhakti Yoga.
In the verse 12, Chapter 12 of Srimad Bhagvad Gita, Sri Krishna says:

श्रेयो हि ज्ञानमभ्यासाज्ज्ञानाद्धयानं विशिष्यते ।
ध्यानात्कर्मफलत्यागस्त्यागाच्छान्तिरनन्तरम्‌ ॥

sreyo hi jnanam abhyasaj
jnanad dhyanam visisyate
dhyanat karma-phala-tyagas
tyagach chhantir anantaram

 

Translation:
sreyah–best; hi–certainly; jnanam–knowledge; abhyasajya–by practice; jnanat–better than knowledge; dhyanam—unrelenting focus and concentration like during meditation; visisyate–especially considered; dhyanat–from meditation; karma-phala-tyagah—to perform required actions without expecting the results of the outcome; tyagat–by such renunciation; santih–peace; anantaram–thereafter.

Answering Arjun’s query while describing Bhakti yoga, Sri Krishna says:
It is best is to do any activity or practice with its complete knowledge, but more than knowledge it is still better to act with your unrelenting focus and concentration.
Above this unrelenting focus and attention is to act without expecting the results of outcome i.e. doing your duty without expecting the fruit of your actions.
For this non expectation gives lasting peace to your soul.

There is another way for achieving lasting peace given by Sage Ashtavakra to his disciple King Janaka in Ashtavakra Gita Chapter 16, verse 11:
Ashtavakra, the sage having eight (ashta) deformations (vakras) in his body says:

हरो यद्युपदेष्टा ते हरिः
कमलजोऽपि वा।
तथापि न तव स्वास्थ्यं
सर्वविस्मरणादृते॥१६-११

Haro Yadyupdeshta te Harih
Kamaljo api va
Ta thapi na tav swasthayam
Sarva vismaranahdryte

Tanslation:
Haro–Lord Shiva , Yadyupdeshta–sermons of knowledge
Harih—Lord Vishnu, Kamaljo api—Lord Brahma
Ta thapi—even then, na tav –your mind will not, swasthayam—attain well being of mind and body
Sarva—all, vismaranahdryte-forgetting or in the state of oblivion to materialistic things

Sage Ashtavakra says, O king Janaka, Even if God Himself (in the form of Brahma, Vishnu or Mahesha) gives you lessons of Supreme knowledge, you cannot attain well being or peace of mind and body. To attain this peace ,you will have to forget all materialistic things from your mind i.e. state of oblivion where no thoughts of materialistic things exist.

The above two sutras (Principals of Life) for achieving lasting peace can be summarized as:

1. Do your duty and perform the required actions without looking or expecting the fruit of those actions.  Sometimes after doing our best of effort, we don’t get the desired results. Krishna says that the outcome of the desired results is controlled by Him. So one should not get disappointed and keep on doing what is required and move further.
2. Don’t keep the thoughts of materialistic things in your mind and remain in a state of forgetfulness. In either of the situations of good and bad, this state is going to benefit.

• If we forget the  losses and bad things happening in our lives, we can save our health from       the harms of stress and tensions.
• Similarly the gains and the achievements in life bring with them the vices like proud, ego,       greed, lust and anger over a period of time. If we forget these also, we will be saved from         the ill effects of such vices which later lead to disturbance of peace of mind.
• To forget, mind need to be engaged in Krishna’s devotional service by chanting His glories, doing NAM sumiran etc.

Hare Krishna