I believe in karma but in a fairly practical manner of understanding that any harm you bring to the world very well has a chance of coming around to haunt you.
Treat somebody like crap, maybe they'll treat somebody else like crap, and that keeps on going down the line until one day, perhaps years later, it so happens you meet somebody who treats you like crap because you set in motion the events of treating people like crap.
Of course, such a direct inheritance is very unlikely: hopefully one person in that chain knows how to shrug off an offense without conducting it to others, and even if not, what are the odds it'd come to you specifically? Not nonexistent, but certainly not high.
What's is a sure bet is just considering that the general state of your society will always impact you, and your influence of the state of the society if through your actions is not completely non-existent. Often it has been observed that the treatment we receive from others around us is often a reflection of ourselves. Karma is not hard to find, if you're looking.
"What goes around comes around" both is and isn't a cosmically remarkable thing. The understanding of karma is beyond the perception of anyone who can't believe something unless it happens right in front of them - to one who does not believe their actions truly have consequence, karma seems silly. However, as any scientist could vouch, for each action there is a reaction, that which has entered the universe does not leave it. Just because the consequences of our actions that we'll actually witness are far less than the ones we will does not mean that any of our actions can be without consequences. That actions have consequence beyond our own perception is both a deep universal truth and something merely obvious.
Karma is not always so profound or difficult. Good Karma is also evident in the smaller things we can do. If you think you're having a string of bad luck, try paying more attention: it's quite possible you could have prevented it if you were more careful. Bad Karma can also come from acts you intended to be beneficial but you didn't put much thought into the lasting consequences of. To take a classic example, give a man a fish, he becomes dependent on people giving him fish to eat, but if you teach a man to fish, he can feed himself and others. Teach too many people to fish, and the fish go extinct, and nobody eats. A life deeper observed bearing in mind the consequences of actions seems to be the solution.
I don't blame you if you can't do this yet - such wisdom takes practice and experience. I'm certainly not perfect at it myself. Look at all this time I wasted on a message board when I could have been doing something more productive that may have improved my life and the life of those around me, like a simple household chore? That's bad karma! Still, if just one person picked up a new understanding as a consequence of my writing, and becomes a better person for it, perhaps it was good karma after all.