Karma, the belief that the actions in your life are somehow tallied and weighed for or against you, falls firmly in the realm of woo-woo. There is no evidence that such a "force" exists, or that there is any entity keeping track of your "sins" so that you may or may not get into some heavenly place.
And yet, I think the concept is useful and effective. Let me explain.
The actions you take do not occur in a vacuum. Everything you do has an impact on something else. If you do something "good," more "good" things are likely to happen because of it. For example, if you throw your trash in a trashcan, others may see you do this and follow suit. Your action has encouraged similar actions. If you throw something out the window of your car, others might see this and decide to do the same.
This is obviously simple, and yet it can be very powerful. If we think of karma as a real force, we can make the world a better place. People will imitate other people, and this will build on itself. Even if you do something out of sight of others, the act of doing influences your behavior, which can influence the behavior of others. At the very least, it can make you feel better about yourself, which is an effect right there.
Of course this isn't foolproof. Nothing you do can prevent others from doing what they want to do, but if we think of "goodness" as a force, exercising that force is liable to increase it.
I'm aware that I've redefined the meaning of "karma," and that I've ignored the judgment aspect of it, but I still believe that acting as though karma exists can lead to good things happening. Think of it as shorthand.
Does this mean I've embraced woo-woo and should be shunned? Not at all. I don't believe in "karma" as defined by religious leaders or dictionaries. I just like the concept as a way to add more "good" to the world.
If you're so inclined, live a day as though karma was real. Pick up a piece of trash off the sidewalk, help an old lady across the street, or buy a stranger lunch. You don't have to believe in mystical powers to make the world a better place, and in this case, the act of belief alone can make the concept true.