You've perhaps heard of the marshmallow test. You place a child in a room and give them two options: They either get one marshmallow NOW or they wait 10 minutes and receive two marshmallows.
Now the long-term studies have been interesting: The children who were willing to wait in order to get more marshmallows had better academic careers, lower drug use (as well as other positive developements) than the children who chose to get one marshmallow immediately. This concept is called delayed gratification. This trait, as you might guess, crucial in achieving almost any goal (regarding fitness, school, business, finance etc.)

My question is: Has anyone read something about how to improve your ability to delay gratification? Any proper psychological studies?