A blog post at The Buddha Is My DJ offers some wonderful ideas for considering karma.
I have come to view karma in the context of cause and effect. In popular-culture portrayals of Buddhism, we do not hear a lot about cause and effect — but in the Zen texts I have read, there are many references to this phrase and its connection to the 12 Links of Dependent Origination.
Everything comes from something, from some confluence of events and conditions.
Once I started getting that connection (ahhh …), karma stopped seeming like simple tit-for-tat.