Lord Krishna was mentioning Arjuna to fight just for the sake of fighting without considering success or failure, so that he may not occur any sin.

So "How?":
One can ask, "How is it possible to fight just for the sake without considering success or failure"? Yes. It is possible as the Lord explains in terms of working without fruitive results. It is popularly called "Working with divine consciousness".

The flowery words of Vedas:

Men of little knowledge read words from the Vedas which recommends various fruitive actions.
Men perform all the actions mentioned in order to get elevated to heavenly planets, to gain power, to have a good health
and so on...
They think only sense satisfaction and opulent life are main goals in life!
For such kind of men, devotional service to the Supreme Lord does not takes place at all even if they try.

Divine consciousness :

Lord Krishna talks about devotional service here. By performing devotional service to the Lord, great men had freed themselves from the results of work. When one's mind frees from the flowery words of Vedas and remains fixed in trance of self-realization by performing devotional service, he has said to be attained 'Divine consciousness'.

Arjuna's query:
Arjuna wanted to know the behaviors of a person who has attained Divine consciousness.

Lord expressed the behaviors of a person who attained divine consciousness:
- He gives up all desires and is completely satisfied in the self alone.
- His mind is not disturbed even amidst miseries or happiness.
- He remains unaffected to whatever good or evil he may obtain.
- He withdraw his senses from sense objects.
- Keeping his senses under complete control and fixed his consciousness on Me.

The Power of senses:
Lord tells us the power of our senses. Senses are so strong that, even if a man tries to control it with full effort, he cannot! It needs strong patience to control our senses.

Why? A simple reason is we contemplate on sense objects. As we keep on looking a sense object, a lust develops
that in turn changes to anger.From anger, our intelligence(Divine consciousness) gets lost. If intelligence gets lost,
our sincere effort to control our mind is wasted!

To summarize:
Thus by following this principle of Divine consciousness, we start experiencing what is real peace!