I've always felt that ethical theories were needlessly complex and tried to explain a lot of things that don't need to be explained. Rather than go through my various problems with famous ethical systems I will lay out mine.
I will start from my belief in game theory and its power to explain human interaction. I believe firmly that any ethical theory that attempts to have humans act "better" than game theory predicts is simply irrelevant. Because we won't. Humans will always do what is best for them.
This would seem to be a very Randian view of humanity and in some sense very hopeless. However, I also believe that humans are uniquely capable of shaping what we value. We are to some extent authors of our desires. With hard work, I can learn to appreciate classical music and value it and pursue it.
Where this comes into play with ethics and game theory is the valuing of making others happy or harming others.
I believe that there are not good or evil actions, only good or evil value systems that create the desire to pursue those courses of action. It is my belief that an evil value system is one that places a positive value on the harming of others, more or less Schadenfreude.
So, for instance, the holocaust was evil, not because of the killing of the Jews, but because the action stemmed from a positive value being placed on the harm of the Jews. Was Hitler more evil than the every day anti-semite who would kill all Jews if he only had the means? I don't think so. Both actions (the done action and the undone action) stemmed from the same evil value system. An evil value system can also stem from placing a negative value on the happiness of others, that is spite is evil. Stealing a letter meant for your sister from her friend, simply to keep her from the joy of reading it, because you don't like to see her happy, is evil.
Good actions are those actions that stem from placing a positive value on the happiness of others. That is, buying my girlfriend flowers, simply because I like to see her happy (not because I think it will get me something later) is a good action because it comes from a good value system. Similarly an action may be good because it stems from a value system that places a negative value on the harm of others. That is risking your life to save another is a good action, if it was done out of genuine care for another's well-being and not for the notoriety.
Neutral actions are those done with no real regard for the harm or happiness of another. That isn't to say that they are completely selfish in the traditional meaning of the term. You can do great things for ethically neutral reasons. If for instance you donate to the poor solely to impress a girl, that is an ethically neutral action. I believe this is the natural, default state of human interaction, the ethically neutral state.
Applying this to game theory, we will look at the central problem to game theory, the prisoner's dilemma.
In the prisoner's dilemma, two individuals are given a choice of two courses of action, for our purposes, we will call them cooperate or take advantage. If each actor chooses to cooperate, lets say they both get a benefit of 10. If they both take advantage, they get 0 benefit. If one cooperates and the other takes advantage, the cooperator will get -5 and the one that takes advantage will get 15. Looking at this, we can see that no matter what the other person does, it is always in your interests to take advantage. If the other person cooperates, you get the bounty of 15. If the other person takes advantage, you at least avoid the negative 5 by yourself taking advantage, and at least get 0 instead of -5.
We can see here that the problem is that when neither person cooperates, they both are stuck with getting 0, when if we could only bind them to cooperate, they'd get 10 each.
This is the most well known problem in game theory, and perhaps the primary reason governments should exist (its my contention that true libertarians are otherwise smart people who don't understand game theory). How it applies to our ethics is by asking the question "what if we could actually make both parties value helping he other party, just for its own sake?"
What if we could change those numbers so that cooperation gets a +5, just because you value helping your brother? Then the math completely changes. Then mutual cooperation confers a benefit of 15 for both. If one person takes advantage and the other cooperates, the cooperater gets 0 and the person taking advantage gets 15. If they both take advantage, they are both stuck at 0. Here, we can interestingly see that its not longer always in your interest to take advantage. By changing the value system, you've made it possible that both will choose to cooperate and thus confer maximum benefit to all.
Robert Wright hypothesized that humanity has been going in this sort of direction throughout history in his book non-zero, the logic of human destiny. His account may be a bit pollyanna-ish, at times, but overall it very much makes sense.
So it is my belief that to lead an ethical life, you should seek to value the happiness of others in all your actions. You should seek to rid yourself of the delight in the harm of others, whenever you may come across it. You should seek to be saddened by the harm of others, whenever you come across it. You should seek to be lifted up whenever you come across another in better spirits than you may be. This will make you a happier person, and make everyone else happier at the same time. Truly a win-win situation, what game theory calls a positive sum interaction. Any ethical theory that is much more complicated than this misses the point.