Karma is a direct contradiction of Christianity. There is no way you can accept the message of Jesus and also accept the idea of karma. Not only is it not compatible with Christian doctrine, but it's just plain illogical.
I'll start by really explaining what karma is defined as. When you really look at it, karma is just about cause and effect. If you do good things, good things will happen to you. If you do bad things, bad things will happen to you. When you apply this to the idea of reincarnation (the origin of the idea of karma), then a person that leads a good life will be reincarnated into a higher state of being, while a person that leads a bad life will be reincarnated into a lower state of being. The way you live your life causes you to either be rewarded or punished.
Whether or not you attribute karma to reincarnation, the idea of "what goes around comes around" is not at all compatible with Christianity. Christianity teaches that we all have done bad things, and we all deserve punishment. We can't atone for our misdeeds by trying to do good deeds. Salvation and eternal life (the reward) is only found through Christ. No person is more or less deserving of the reward.
Christianity believes in grace, not karma. They are two completely contradictory ideas, and you cannot logically believe in both. Karma says you get the reward or punishment that you deserve. Grace says you get the reward even if you deserve the punishment.
So when you're giving your waitress a big tip, do so because you want to be gracious, like Christ, not because you want to rack up some "good karma" in hopes that you'll somehow benefit later down the road.
"The beauty of grace is that it makes life not fair." -Be My Escape by Relient K