Tithing in the New Testament
Tithing in the Old Testament meant setting apart a tenth of your possessions unto God. Most churches teach that Christians today should give 10% of their income, and I've even seen some church websites list tithing in their "What We Believe" section right next to critical doctrines such as the Trinity.
"Thou shalt truly tithe all the increase of thy seed, that the field bringeth forth year by year. And thou shalt eat before the Lord thy God, in the place which he shall choose to place his name there, the tithe of thy corn, of thy wine, and of thine oil, and the firstlings of thy herds and of thy flocks; that thou mayest learn to fear the Lord thy God always. And if the way be too long for thee, so that thou art not able to carry it; or if the place be too far from thee, which the Lord thy God shall choose to set his name there, when the Lord thy God hath blessed thee: Then shalt thou turn it into money, and bind up the money in thine hand, and shalt go unto the place which the Lord thy God shall choose." - Deuteronomy 14:22-25
Tithing (giving 10%) is actually nowhere taught in the NT as a requirement for Christians. However, we are definitely taught to give! John Wesley had the following to say about giving:
"Do you not know that God entrusted you with that money (all above what buys necessities for your families) to feed the hungry, to clothe the naked, to help the stranger, the widow, the fatherless; and, indeed, as far as it will go, to relieve the wants of all mankind? How can you, how dare you, defraud the Lord, by applying it to any other purpose?"
The most famous passage in the NT regarding giving is in 2 Corinthians 9:7 which says, "Every man according as he purposeth in his heart, so let him give; not grudgingly, or of necessity: for God loveth a cheerful giver." We should not give because we feel obligated to, or that it will "make God love us more" but because the Holy Spirit moves us to do so, and because we realize that everything we have came from God to begin with, and we are just stewards passing through this life.
In 1 Corinthians 9 (and elsewhere) we are told that spiritual leaders, those who share and teach the gospel, should be compensated for their labor. Paul writes, "Even so hath the Lord ordained that they which preach the gospel should live of the gospel." Therefore, I strongly believe that we should send money to those who are sincere and bold in preaching the Word of God. However, we definitely have to be careful about where our money goes because there are plenty of frauds out there.
Do not stint yourself, like a Jew rather than a Christian, to this or that proportion. "Render unto God," not a tenth, not a third, not half, but all that is God's, be it more or less; by employing all on yourself, your household, the household of faith, and all mankind, in such a manner, that you may give a good account of your stewardship when ye can be no longer stewards; in such a manner as the oracles of God direct, both by general and particular precepts; in such a manner, that whatever ye do may be "a sacrifice of a sweet-smelling savor to God," and that every act may be rewarded in that day when the Lord cometh with all his saints. - John Wesley
Finally, I want to add that church leaders with mansions, new cars, and that fly first class should probably not get your money. A pastor that's a millionaire is almost certainly not a man of God.