I found a huge number of hits on my site where users look for this answer. I thought I’d just write up a quick note.

Can positive thinking change your karma?
Short Answer: Yes, but it doesn’t mean that your past karma goes unaccounted for.

Long Answer…
Karma means action.  And by positive thinking you have already acted in a way that will help you.
But I’m going to assume that you meant “Can it change my past karma ( either past life or current)?” The answer is: Yes only after you have completely paid back your previous karmic debts.
Karmic debts are usually no different from the financial debts in our little Wall street world. There is one exception; there is no bankruptcy law here. You cannot bail out without payback. The only good news is that your payback can be negotiated in any form acceptable to the other person and yourself. If you are reading this, the negotiation you are thinking about has already happened in the astral before birth (if you consider time as a linear). It will most probably not happen in the way you might think right now but the opportunity will show itself when its time. Act then. Act wise, rationalize and see compassionately. This is the crux and core of positive thinking.

How do I know this?
I can only speak from personal experience. My current life karma becomes obvious over time. One instance I remember clearly was a scene on the Basketball Court. I remember falling and tumbling on the court after being tripped by a newbie during a layup. This immediately took me to the time I was a newbie and I did the same thing unintentionally to someone else. I remember him tumbling and falling the same way.  For my past life karma, I cross check with my wife’s help. She astral projects into our past lives to verify the payback. Since not everyone has this privilege, the only way is to go with the flow and act wisely and rationally (and this is obviously universal advice.)