Caring for your future self
One of the lessons I constantly find myself relearning is how to prioritize -- and care for -- your future self. This may mean choosing salad over lasagna so your future self doesn't fall asleep while trying to hit a deadline.
It may look like caring for your body by going to the gym three days a week, like my 94 year old grandfather! (3 miles on the bike + 5 weight machines -- he's an inspiration!)
It may mean working before playing, so that you can be stress free and enjoy yourself fully. Or giving yourself enough time to actually accomplish your goals (rather than set them so lofty that you inevitably fail).
This lesson is really a framework for living: Value your future self as much as your current self.