Malachi 3:8 (NKJV)
"Will a man rob God? Yet you have robbed Me!" But you say, 'In what way have we robbed You?' In tithes and offerings....
 
R.C. Sproul wrote a great article on this verse that I'm summarizing here but I recommend reading Sproul's article also http://www.crosswalk.com/spirituallife/11607311/page1/.
 
God is saying that to withhold the full measure of giving God expects of His people is to be guilty of theft against God Himself resulting in a curse on the thief. The principle in the Old Testament was to give 1/10th of our gross yearly income to the temple. But today, one statistic shows that only 4% of Christians actually give 1/10th - only 4%. But regardless of the accuracy of this percentage, it is obvious that the vast majority of Christians do not tithe.
 
The most common argument against tithing is that tithing is part of the Old Testament and no longer applies to us. But nowhere in the New Testament is tithing said to be ended. In fact, Jesus contrasted the Old and the New when He taught "the Old covenant says don't commit adultery but the new says don't even allow lust in your heart, much less adultery". The Old is only a shadow of the New, and that which casts the shadow is GREATER than the shadow. So, tithing should be the starting point of even greater responsibility, not less. But most of us give by far less than the believers under the Old Testament.
 
The second argument is that we "can't afford to tithe". But this puts the idea of giving to God at the bottom of a list of priorities instead of at the top. God doesn't want to be at the bottom of my priority list and when I am willing to "sacrifice" to make sure I have cable TV but I'm not willing to prioritize God's work, that can be a really weak argument. Besides, in America, down economy or not, we have so much food we have to worry about weight, so much housing we can afford to think about a bigger home (when more than ½ the world lives in the equivalent of a card board box), and so much access to transportation we can worry about getting a new car every 2 or 3 years when more than 50% of the world has none.
 
Finally, prioritizing our giving to God requires us to budget, plan and be purposeful with our resources instead of being poor managers. Better management is going to be blessed. In addition, when we give faithfully we find a joy in really helping God's work that we can't find as outsiders. But most importantly, God Himself said "Put me to the test, says the LORD of hosts, if I will not open the windows of heaven for you and pour down for you a blessing until there is no more need" (3:10). Have you put God to the test in your giving? Maybe it's time you started.