A lot of people go through life with almost every thought on what others say or think.
We have a lot of assumptions throughout the day, pay attention to your thoughts.
Most of the time it is whether the thing you say, think, act will be taken in a good way by others; How others will perceive it, 'are others are doing the same'?

We have a strong survival instinct in our brain dedicated to that (*below explanation). This mechanism makes sure we are aligned with our tribe, it is hardwired in our social brain.  

There are soul-free spirits that seem as if they do not give a fuck. They look always secure and confident, in their body language, words and thoughts. They react in peace and calmness to almost any situation, conversation or debate. I call them people with Zen. I worked with a senior level manager in a big corporate who always behaved this way, even in the most stressful times.

Here are ways that will help you in deciding which things deserve a fuck and which will only be a waste of energy. 

One way is to tame the Ego.
Second  is to grow your own self-worth and from there – self confidence.
Third  is to distinguish the wheat from the chaff. 
Forth  Non attachment and minding our own business.

1.       Tame the Ego
How do we tame our ego? It’s a subject for a whole new post :) I will probably elaborate on it another time, but the basics can be depicted shortly:
First you need to notice it, whenever you have a reaction, is it from the ego? Be mindful and aware.
After you are aware of your own ego and when it arise, you can become friends. If you try to fight it – it will probably win, so don’t come to it from resistance.
But moving from friends to boss…
Stop and take a ‘pause & thought’ break instead of reacting automatically to a situation or a saying. During this time of pause, decide if to react from your ego or from a calmer place.
Be in control of your thoughts, reaction and ego.

2.     Self-worth
There are not enough words to describe how imperative this is. All of our negative patterns, limiting believes, assumptions, reactions, emotions, behaviour and life experiences are caused by the disease of not having self-worth. I call it disease because its implications are devastating.
Build your self-worth. know what you worth. Know that you are worthy. Know that there is a reason you were put on this planet. Know your goal, your purpose in this life. Know who you really are. Only then – not giving a fuck will come as easy as breathing. You know why? Because you rely on yourself, you know what’s good for you, you know what to go away from. You have explored and found and connected to your inner self and you know that you matter.
Self-worth is followed by self-confidence and with it – the peace & equanimity that will accompany you onward.

3.     Distinguish the wheat from the chaff

Of course when we say we want to not give a f**, there are still things in life we do wish to attend to and give our love and care for. It doesn’t mean that we want to be careless robots.
The universe is vast – our problems are small in comparison. Try to see what worth your energy, and what’s not. Choose your battles. It sounds like cliché but clichés are often true.
If a thought about other people or someone else makes you think crazy and obsessively, or just doesn’t serve you, ask yourself – how will I feel without it? Will it liberate me and I feel free? Will a weight get off my shoulders? If the answer is yes- this is your choice. This is a battle to drop.
Immerse yourself into things that really matter, be focused on the goal (which you have found in the previous section).
When your energy is taken by the more important things – it will bring you more results, you will be in a flow.

4.    Non Attachment & minding your own business

 Buddhism says don’t be attached to anything, not good praise nor bad feedback.
Think about it, do you need the praise? You already know what you worth :) Do you need the negativity? You already know what you worth :)
Don’t let the ball be in their court, don’t live according to the reaction/response of other people. Don’t be afraid of their believe system and opinion. It is theirs, not yours.
However they react to what you say – is non for you to take, you can leave it with them.
People act from different frequencies of energy, sometimes you are not in the same frequency.
Mind your own business and always remember they are driven from their business, it is not necessarily or entirely about you.

 

I have to admit, it is very hard to practice all the techniques I mentioned.
The work is continuous, never ending, and VERY rewarding.

*Explanation about our Social Brain: 
"Humans are hardwired to connect with others, and our brains have adapted a nifty way to make sure we do. We have specialized brain cells called mirror neurons - whose sole purpose is to keep track of what other people are thinking, feeling, and doing. These mirror neurons are sprinkled throughout the brain to help us understand the full range of other people's experiences.  ...Our sense of self depend on our relationships with others, and in many ways we only know who we are by thinking about other people. Because we include other people in our sense of self, their choices influence our choices. " From the book: The willpower instinct" by Kelly McGonigal, Ph.D.