Show me the money

Had a good discussion with our young marrieds group last night about tithing. The lazy side of me would prefer to know our church's policy so that I can just follow some clear-cut rules. But the thinking side of me knows it can't be that easy. Giving is personal. It's between you and God. And if you're just following some rules you're really not speaking from your heart when you make your offering.

And then I think about poor Cain. Why was his offering rejected? Some say, "Oh, well his heart must have not been right," or, "God prefers meat/sacrifice." Bringing grain from the earth is no easy task, so I know that Cain worked really, really hard to bring that offering to the Lord. I guess I'll never know why God didn't like it as much as Abel's. Which brings me back to my own questions. How do I know that what I give and offer is worthy of God's acceptance? And I suppose the answer is that Christ gave the ultimate sacrifice, and covered my paltry offerings with his life. So maybe as long as I'm giving out of joy, thankfulness and praise then God's happy? But if I'm giving out of obligation, and trying to earn God's love through works, then He's not happy?