Peace of mind is a kind of emotional and mental calmness, with no fears, worries and stress. In such a situation, your mind would remain quiet and it would experience a true sense of happiness and freedom.
Such a joyous and peaceful moment are not very rare. You might have gone through it in the past, mostly during those times when are engaged in some activity and also in other absorbing and interesting activity.

What you can actually do to keep your mind engaged and free from worries are as follows:
1. Keep yourself away from all sorts of negative conversations and negative people around. Do not ever allow their words and thoughts hamper your mind. It should not be allowed to affect your moods and state of mind.
2. Cultivate within yourself the habit of forgetting and forgiving. Do not hold grudges in your mind which could nurture more ill feelings that will keep on hurting you every now and then. This will cause lack of sleep too.
3. Do not be jealous of others. Showing off jealousy means that you have lower levels of self-esteem and consider yourself interior to others. These negative feelings such as low self-esteem and jealousy would give rise to lack of peace of mind.
4. Meditation is not everyone's cup of tea. But if you really want to try and are willing to do it, spare sometime for it so that you become more relaxed and happy.