Most important life lesson is from Tony Robins - not to react to what others say or think about me and to situations that are beyond my control. It is our tendency to react to all kind of people and situations in a negative way and this affects our emotion and our life. Once I realised that all these are beyond my control, hence I should not have any reaction towards it, my emotion is much better and life in general improves for the better.
Next important lesson is compounding interest. It is the biggest secret in making free money!