How can I stop myself from thinking too much?

One of the best books I've read on this topic 'The power of NOW by Eckhart Tolle' in which he has shared one of his life changing experience where out of frustration he told himself, "I had it enough, I just cannot stay with myself any longer". 

This made him realize that there are two 'Eckharts' in him. One is the 'I' and the other is the 'Self' that the former cannot stay with & is highly fed up too.

He thought that perhaps only one of them is real and other is fake.

I learned a lot from this. Firstly, we have two minds in us. Subconscious mind that does all the monkey like chattering of thoughts in the background and the conscious mind that is used in real time for the purpose of doing the tasks at hand.

Here are a few things you can do in order to calm the mental chatter down: 

1. Focus on your Breathing whenever you feel lost in thoughts, breathing instantly connects you to the present moment
2. Just grab your cell phone and put a time of 30 seconds. Just focus on the black color that you see when you close your eyes for these 30 seconds. Do this a few times in a day and you will have a better control of your thought flow
3. Be careful of all those moments when you are complaining, it carries an unconscious negative charge, so make sure you complaint less 
4. Do not dwell on the grievance too much, either change it, seek help to change it, if nothing works out then simply accept it, BUT do not dwell on it unnecessarily, there is absolutely nothing that you will gain out of it
5. Listening to the sounds of the environment around you for not more than a minute will snap you back to the present moment and that will in turn drastically reduce the mental chatter 
6. Listening to Nature Sounds while working helps tremendously. Some studies suggest that nature sounds boost productivity too
7. Finally a little philosophy to end with, the key to reduce or stop the chattering of unnecessary thoughts is to enjoy what you do and do what you enjoy, when your mind is at peace with the activity that you are doing, it will enjoy the present moment and will seldom think of the past or future. But when you force yourself to do things, boring enough to put you to sleep, then your mind will begin to visit the past and create fantasy based future to generate some interest