Will power is a popular concept for spiritual folks. If you enjoy pornography, exchange spiritual fantasies in chat rooms, cheat on your spouse, etc., you probably regret your action afterwards. You promise yourself that in the future everything will get better. You will overcome your dirty urges with the help of willpower, won't you? There is a big problem with the will power concept. Who is the authority that enforces this concept? It is your mind, is it not? This concept has conflict written all over it. The authority that sends you on the ego trips in the first place is also the authority that tells you afterwards that you shouldn't have done this egocentric action in the first place. You make the thief police chief and judge as well.
Mindfulness is the ability to monitor the inner workings of your mind. Catch negative thought patters, observe inconsiderable things you say or do. Spot the ego at work when you talk to the cab driver differently than to your boss. See how your mind goes off to sleep when you act compulsively. And to go back to the earlier described sex struggle, find the reason why you can't enjoy the sexual life with your partner and need those other outlets of your repressed sexual energy instead.
J. Krishnamurti advocates this monitoring process in his work, and the longer I study myself and the problems we are facing, the more I realize that observing is all we can do. The mind, with all its experience and psychological pitfalls can't be trusted with anything other than analytical problem solving. Luckily, when we are really silent, or really going with the flow, there is an authority that guides us. That is all we can do, be mindful of things that are going on in our head and outside of us.