What's in YOUR wallet?
If a preacher asks you that question, tell them "It's none of your business". Seriously.
The Tithe (10%) was for Israel and no one else. That Abraham gave a tenth of the spoils of war to a mysterious OT priest named Melchizadek does not "prove" that it is a command for Gentiles. Abraham was, after all, the father of the Jewish nation, not just anyone. God told this same Abraham to begin the practice of circumcision, which was long before Moses and the Law. If we must tithe based on the fact that Abraham did, then we are also bound by circumcision and everything else. And there is no evidence that tithing was Abraham's common practice, only that he tithed the spoils of war one time.
Likewise, the oft-misused book of Malachi is an indictment against the nation of Israel, not the whole world. While it is true that God withholds blessing from his people (Jew or Gentile) due to disobedience, the command to tithe does not apply to anyone but Jews. The context is very specific. And the Jewish tithe was not in money, but in produce and livestock. Try taking some tomatos and chickens to church next Sunday! Plus, there is no Temple into which these things are to be brought, no priesthood who depends upon them to live. Jesus and Paul said that we, the believers, are the temple of the Holy Spirit, not some building.
So why oh why do so many preachers get worked up about tithing? "It's the economy, stupid". In other words, the "church" has buildings to maintain, and salaries to pay, and supplies to stock, and Sunday School books to order (how in the world are all those exegetically-challenged teachers supposed to teach without "quarterlies" that pre-digest everything for them?). We don't hear sermons on tithing because of the poor and needy, but because we have bills to pay.
Hence the guilt trips, the proof texts, the record keeping, the statistics. You can't go to church for very long without hearing a breakdown of what groups of church members give what miniscule amounts and how unspiritual the people must be. Shame upon them for not forking over more money so the choir can have new robes! Shame upon them for having a nicer house than the Pastor!
If those same preachers would concentrate on actually teaching the teachers, on expositing the scriptures so the people know what the Bible actually says about giving, and on the fact that everything we have and are belongs to God not just 10%, then maybe the guilt trips wouldn't be necessary. If they'd wake up to the unscriptural structure of the traditional church and realize we don't need buildings at all, or choirs (don't get me wrong, I like choirs and bands as much as anyone, but the church's money shouldn't be supporting them), then I can guarantee we'd hear a lot less pulpit-pounding about legalistic tithing.