Nothing in the league of biggest life lesson, but somethings which I am happy about (or trying work-in-progress):

-- Cut down all sugar. I started to take my tea/coffee without sugar. And no sweets. Try to take a fruit when have the urge. Which resulted in a drop of 2.5 kgs of weight (better BMI)

-- Realizing that yelling at family members (e.g. kids) make you unhappy later on for a long time, with guilt/etc. Better at it, but its obviously a perennial work-in-progress

-- Off Facebook, for some months. Always had the power to quit and had quit intermittently couple times before. But the last stretch I was fooling myself that I need to be on it, to share interesting stuff with some friends I like. (Now when I really have an urge to share, just bcc select friends in email. But this mode and thought process may change again.)